Soil microorganisms play critical roles in ecosystem function.  However, the relative impact of the potassium (K) fertilizer gradient on the microbial community in wheat‒maize double-cropping systems remains unclear.  In this long-term field experiment (2008–2019), we researched bacterial and fungal diversity, composition, and community assemblage in the soil along a K fertilizer gradient in the wheat season (K0, no K fertilizer; K1, 45 kg ha⁻¹ K₂O; K2, 90 kg ha⁻¹ K₂O; K3, 135 kg ha⁻¹ K₂O) and in the maize season (K0, no K fertilizer; K1, 150 kg ha⁻¹ K₂O; K2, 300 kg ha⁻¹ K₂O; K3, 450 kg ha⁻¹ K₂O) using bacterial 16S rRNA and fungal internally transcribed spacer (ITS) data.  We observed that environmental variables, such as mean annual soil temperature (MAT) and precipitation, available K, ammonium, nitrate, and organic matter, impacted the soil bacterial and fungal communities, and their impacts varied with fertilizer treatments and crop species.  Furthermore, the relative abundance of bacteria involved in soil nutrient transformation (phylum Actinobacteria and class Alphaproteobacteria) in the wheat season was significantly increased compared to the maize season, and the optimal K fertilizer dosage (K2 treatment) boosted the relative bacterial abundance of soil nutrient transformation (genus Lactobacillus) and soil denitrification (phylum Proteobacteria) bacteria in the wheat season.  The abundance of the soil bacterial community promoting root growth and nutrient absorption (genus Herbaspirillum) in the maize season was improved compared to the wheat season, and the K2 treatment enhanced the bacterial abundance of soil nutrient transformation (genus MND1) and soil nitrogen cycling (genus Nitrospira) genera in the maize season.  The results indicated that the bacterial and fungal communities in the double-cropping system exhibited variable sensitivities and assembly mechanisms along a K fertilizer gradient, and microhabitats explained the largest amount of the variation in crop yields, and improved wheat‒maize yields by 11.2–22.6 and 9.2–23.8% with K addition, respectively.  These modes are shaped contemporaneously by the different meteorological factors and soil nutrient changes in the K fertilizer gradients.