The main objective of this paper is to investigate the conversion efficiency of actual operation photovoltaic (PV) inverter in Kunming. Additionally, this work can be a demonstration to indicate efficiency loss of PV inverter used in different region with different solar radiation levels. The DC and AC data of a 500KW grid connect inverters were recorded in 2012, and three kinds conversion efficiency (instantaneous, energy and average conversion efficiency) were used to evaluate the inverter’s performance. Its performance was analyzed in three typical weather conditions: clear day, cloudy day and somber day. The results show that the inverter has high conversion efficiency when the DC input close to the rated input power, and has much lower conversion efficiency when the DC input is very low. For actual operating, the conversion efficiency is really low which does not reach 90%, when the input is less than 50kW,, and the limit efficiency of this inverter should be 96.00%. Correspondingly, PV inverter performs excellently in noontime of clear day, and is passable in cloudy day, but has a inadequate performance in somber day, morning and evening.