Plastic mulching film plays an important role in development of agriculture. However, a series of problems such as pollution of plastic film residue left in the field occur. It is required to make a reasonable assessment on film mulching and rationally utilize it. Based on its application, this study proposed a concept of crop safety period of plastic film mulching, and its estimation methods. Crop safety period through plastic film mulching means the appropriate choice of a photo-selective mulch film according to the transplant season, to the pedoclimatic conditions of the area under cultivation and, finally, to the type of crop. In the days before the transplant, the plastic mulching film should be basic whole, and it works to decrease soil moisture evaporation, to warm soil and to prevent weeds. Successively, these functions almost reduce. The selection of crop safety period of plastic film mulching is helpful to manage agricultural activities efficiently; guiding film makers to produce both meet the demand of production and lower cost of plastic products; to help farmers due to the types of crop and production conditions choosing appropriate plastic mulch film. Based on measurement of function of plastic film mulching, and crop canopy density, 2 kinds of method were established. The first one is to continuously monitor soil moisture，temperature and weed control, to build relationship between the function of soil temperature, moisture, weed suppression and mulching treatments, with that to seek the intersection of them. The coincidence point means functions disappear of plastic mulching and the days from starting mulching to this date is a certain crop safety period in a certain site. The second is crop canopy density to determine crop safety of plastic film mulching period, which includes 2 steps: to measure the change of crop canopy density, and the functions (soil temperature and moisture etc.) of mulching under no mulching and mulching at same time, to establish the curve of crop canopy density and the main function of plastic film in order to cross the best condition between crop canopy density and the contribute of mulch films aimed to the determination of the ideal crop safeguard period. All in all, plastic mulching film has been widely used in global, especially in China, in the past 30 years, and its application area and efficacy works better than any other agronomic practices. Concept of crop safety period of plastic film mulching has been firstly proposed in this paper, and further research in the connotation and extension of its definition still need to be developed. Moreover, the mentioned estimation methods around crop safety period of plastic film mulching showed some usefulness in cotton in Xinjiang, corn in Northeast and tobacco in southwest planting, but more extensive research, application and improvement were needed by far. The findings from this research provide an important information base for improving adaptability evaluation system of plastic mulching film, investigating the spatial pattern of plastic mulching film utilization, and biodegradable plastic film production and application.