Abstract: Field experiments were carried out to analyze the reasons for maize yield decrease under plastic film mulching conditon. The results show that plastic film mulching significantly increases soil temperature, promotes maize growth and significantly speeds up phenostages of maize. However, maize was encountered with serious summer drought at the heading stage. The normal pollination of maize was affected seriously, which resulted in the decrease of maize yield. Maize yields with plastic film mulching were 73.6% and 12.9% lower than that of open-ground planting for two consecutive years. The frequency of serious summer drought was 15.9% at the maize heading stage in experiment area, which might resulted in maize yield decrease. These facts should not be ignored and need investigating. So plastic film mulching should be rationally chosen and applied in semi-arid areas according to concrete conditions.