Abstract: The aims are to explore the limiting factors in the constituents of biogas slurry affecting farmland safe digestion, and disclose the mechanism influencing crop growth. The effects of COD in biogas slurry on soil environmental factors and cucumber seedling growth were monitored by micro-zone test, and the correlations between COD amount, seedling growth, soil nutrients and other ecological factors were analyzed. The results showed that 1566 kg/hm2 COD promoted the growth of seedlings, accelerated the appearance of available phosphorus peaks, and had obvious effect of water retention and heat preservation. 3132 kg/hm2 COD inhibited the growth of seedlings, postponed the supply peak of available phosphorus. The changes of soil moisture, temperature, and electrical conductivity were not conducive to the seedling growth. The correlation analysis showed that the COD in biogas slurry had a parabola correlation with seedling dry matter and N, P and K accumulation of plant. According to curve fitting, the optimal application safety threshold of COD was 1102- 1442 kg/hm2 and the maximum application safety threshold was 2208-2884 kg/hm2. COD amount is the key limiting factor affecting the utilization of biogas slurry on farmland under the condition of equivalent NPK fertilizer, it should be considered when farmland is used as a safe and efficient method to digest biogas slurry.