Abstract: The effect of plant growth regulator mepiquat chloride (1,1-dimethyl-piperidiniuchloride, DPC) on cotton epidermal wax deposition and photosynthesis were investigated using ‘Xinluzao 55’ as the sample. DPC was respectively sprayed on the foliage at the doses of the active ingredients of 0, 4.5, 7.5, 10.5 and 15.0 g/hm2. The photosynthetic rate and wax content of cotton leaves were measured at 1, 4, 7, 12 days after the treatment. The SPAD value of leaves was also tested within 12 days after the application of DPC. The result showed that after DPC was sprayed to cotton leaves, the photosynthetic rate of the cotton leaves, the wax content and the SPAD value have definite reliance on DPC concentration and the processing time. The photosynthetic rate of cotton could be improved by DPC treatment (4.5 g/hm2 or 7.5 g/hm2) and this effect could last for a long period. With the increase of the treatment time and DPC content, the SPAD value of cotton was increased, which indicated that the chlorophyll content of cotton leaves increased and leaf color deepened. The concentration of 4.5 g/hm2 was more favorable for the wax deposition of cotton leaf epidermis. The correlation analysis of photosynthetic rate, photosynthetic rate and SPAD value showed that the photosynthetic rate had a significant positive correlation with the wax and SPAD values. It illustrated that the deposition of wax and the increase of chlorophyll content were beneficial to the photosynthesis of cotton leaves and enhanced the resistance.