Abstract: Cotton is an important economic crop in China, the stable development of cotton production is concerned with the interests of 20 million of farmers. Yield is the most important trait on the premise of the comprehensive development of other traits, so high yield is the most important goal of cotton planting. From 1950 to 2015, the cotton yield per unit area in China was increased by more than 9 times, and the introduction and improvement of varieties have made a great contribution to the increase of cotton yield. In recent years, the improvement of cotton yield per unit area was slow because of the narrow genetic diversity, hindering seriously the development of cotton industry. Cotton yield per unit area contains four major components, stock number per unit area, boll number per plant, single boll weight and lint percentage. This paper analyzed the roles of four components in cotton planting. Boll number per plant and lint percentage are more important for the cotton yield breeding, but the improvement of cotton yield is a complex progress which need every trait coordinated properly. At the same time, this paper discussed the important ways and its research progress to be used for high-yield breeding. Germplasm introduction has played an important role in raising cotton yield and replaced the Gossypium arboreum varieties of lower productivity and poor quality in China, and promoting the development of cotton breeding in China. Some significant varieties were cultivated by conventional breeding, for example, early maturing cotton CCRI 16, high-yielding cotton variety Lumian1, high-yielding and disease-resistant variety CCRI 12 etc. These varieties promoted the development of cotton industry in China, and which were the important parents of cotton breeding. The utilization of heterosis is an effective approach to cotton breeding of high yield and good quality. Lots of significant hybrids were cultivated by cross breeding. For example, the planting area of CCRI 29 once accounted for 50% of hybrid cotton in Yangtze River basin of China. And the research of male sterility line plays a key role for sustainable development of heterosis utilization. The development of molecular marker technology provides technical supports for molecular breeding of cotton, and multiple stable QTL related with yield trait has laid a foundation for molecular mark assisted breeding. The development of transgenic technology provides an opportunity for cotton molecular design breeding. The discovered genes related with yield trait are few, so more work should be done on the excavation of new genes. At present, the cotton yield per unit area in China is at the international leading level. Because China has more people and less land, the increase of cotton yield per unit area is a way out for cotton industry based on the premise that not to reclaim land under food crops. So the following suggestions were put forward: It is necessary to collect plant resources and pay more attention to the innovation of germplasm resources; the cytoplasmic male sterility (CMS) research should be strengthened, the seed production technology and cost should be simplified and promoting the potential of hybrid breeding; exploring the high yield related genes using high-throughput sequencing technologies in whole genome for molecular marker assisted breeding and genome-wide selective breeding; it is significant to cultivate new varieties with high-yield, good quality, early-maturing and suitable for mechanization planting with pyramiding breeding.

Key words: cotton, yield trait, cross-breeding, heterosis, molecular breeding