The distribution and variation of phosphine concentration in different layers and surface of grain heap, outlet of the squat silo and the pipes of the circumfluent fumigation system were tested by medicating on the surface of grain heap and bilateral circumfluent fumigation. The distribution diffusion rule of phosphine in grain bulk was analyzed. The results showed that the concentration of phosphine in the grain heap of the silo reached a uniformity of 0.6 at sixth day. Although the concentration of phosphinein both sides of the loop pipe reached to over 200 mL/m3 in only 1 ~2 days, while lower than 200 mL/m3 at the bottom outlet of the silo after eight days. Therefore, it is not enough by just detecting the concentration of phosphine in the pipe and on the surface of the grain heap to guarantee the efficacy of fumigation, since it needs quite a long time to achieve uniform of the concentration, which is of great significance for guiding the circumfluent fumigation in the shallow silos and ensuring the effect of fumigation and killing insects, and has great practical value.