Polyaspartic acid (PASP) is a nontoxic, biodegradable, environmentally friendly polymer and is widely used as a fertilizer synergist in agricultural production.  In many old orchards and vegetable gardens, highly fertile soil is often accompanied by severe heavy metal contamination.  The present study was designed to investigate mineral element interactions mediated by PASP under copper (Cu)+cadmium (Cd) combined stress to provide reasonable suggestions for scientific fertilization.  A pot experiment was conducted in which tomato seedlings served as plant materials.  A concentration of 700 mg L^–1 PASP and foliar spraying application methods were selected based on previous experiments.  Four treatments were applied: normal soil (control (CK)), Cu+Cd (combined stress), Cu+Cd+PASP, and normal soil+PASP.  The plant biomass, root activity, and mineral elements were measured, and these data were analyzed by Data Processing System (DPS) statistical software.  The results showed that, under Cu+Cd combined stress, PASP promoted stem diameter growth, root activity and chlorophyll content and ultimately increased the biomass of tomato seedlings to different degrees.  Moreover, the content of both Cu and Cd and their individual accumulation in plants decreased.  PASP increased the distribution of Cu and Cd in the roots under Cu+Cd combined stress, and the translocation ability from the roots to shoots was significantly restricted.  With respect to essential elements, PASP promoted mainly the absorption and translocation of potassium (K), calcium (Ca), and magnesium (Mg), which greatly exerted physiological roles.  However, the variation trends of Cu and Cd under normal soil conditions contrasted with those under stress conditions.  With respect to essential elements other than K, Ca, and Mg, PASP mostly restrained their absorption but promoted their translocation.  The regulatory mechanism of PASP differed between the combined stress conditions and normal soil conditions.  Under the combined stress conditions, PASP seemed to mainly promote these advantageous factors that exert physiological regulatory functions.  Under normal soil conditions, PASP mainly acted as a biological stimulant or signaling molecule for increased nutrient efficiency, which caused greater biomass productivity.