Abstract: A reasonable co-growth period between intercrop species can effectively improve the use efficiency of soil resources in time-space fabric. However, sowing date of intercrop plants directly determines the length of co-growth period of intercrop plants. This can directly affect intercropped plant productivity and interspecific interaction due to niche differentiation over time. Thus, a study was carried out to determine the effect of sowing date of intercropped maize on yield, productivity and interspecific competitiveness of crops of soybean/maize intercropping system. A total of three sowing dates of maize were set up, which were 24^th April (M1, co-growth period of maize and soybean was 165 days), 4^th May (M2, co-growth period of maize and soybean was 150 days) and 14^th May (M3, co-growth period of maize and soybean was 140 days). Then yield and dry matter accumulation of crops were investigated in both intercropping and sole systems, and system productivity and interspecific competitiveness also analyzed. The result showed that yield of soybean/maize intercropping was not affected by maize sowing date. Land equivalent ratio (LER) of all intercropping systems was greater than 1. However, LER value decreased with delay of sowing date of intercropped maize. The largest LER was for M1 (1.37) treatment. There were no significant differences in yields and yield components of intercropped soybeans with different maize sowing dates. Yield of intercropped maize and productivity of intercropping system decreased with delay of sowing date of intercropped maize. The 100-grain weight of intercropped maize decreased with delay of sowing date, 100-grain weight of M3 (26.1 g) was 71% that of M1 (36.6 g) treatment. The competitive ability of intercropped maize decreased during later co-growth period due to delay in sowing date. During early co-growth period, the competitive ability of soybean for growth resources was stronger than that of maize. However, in later co-growth period (from September to harvest), the competitive ability of maize significantly increased and was greater than that of soybean. However, the aggressiveness of soybean relative to maize (A_sm) under M3 was significantly higher than those under M1 and M2. Meanwhile, relative crowding coefficient of maize also was higher than that of soybean. This decreased with delay in sowing date of intercropped maize in the order of M1 > M2 > M3 and with competitive ratio values of M1 > M2 > M3. Therefore, the most reasonable sowing date of maize in soybean/maize intercropping system in Hexi Corridor of Gansu Province was on 24^th April. Here, yield of intercropped plants and the productivity of intercropping systems were maintained. The delayed sowing dates of maize decreased yield and productivity of intercropping system.