Thermal performance of phase change material (PCM) glazing unit in summer under different climatic conditions is studied by numerical simulation in this paper. Through analyzing the data of meteorological stations in all provinces of China, the law that strong solar radiation (SR) will be accompanied by large diurnal temperature range (DTR) and weak SR will be accompanied by small DTR is discovered. Taking the inner surface temperature of glazing unit ($T_{sur}$) and total heat flow entering indoor side through glazing unit ($Q_{total}$) as the evaluation indexes, the applicability of PCM glazing unit under twelve different working conditions of strong SR with large DTR and weak SR with small DTR is analyzed. The results show that, from the point of view of $T_{sur}$, PCM glazing unit has better thermal performance than ordinary double-layer glazing unit under all working conditions ($T_{sur}$ is reduced by 1 \% - 19 \%). In addition, compared with the conditions of weak SR and small DTR ($T_{sur}$ is reduced by 6 \% on average), PCM glazing unit is more applicable under conditions of strong SR and large DTR ($T_{sur}$ is reduced by 13 \% on average). From the perspective of $Q_{total}$, PCM glazing unit still shows a better thermal performance than ordinary double-layer window unit ($Q_{total}$ is decreased by 4 \%-80 \%). Different from the conclusions when $T_{sur}$ is used as the evaluation index, PCM glazing unit seems to be more suitable for the condition of weak SR and small DTR ($Q_{total}$ is decreased by 35 \% on average) rather than strong SR and large DTR conditions ($Q_{total}$ is decreased by 26 \% on average). This may be the result of inappropriate thickness of the PCM layer in this simulation, resulting in insufficient heat storage capacity of PCM glazing unit to meet the intensity of SR. Therefore, the heat storage capacity of PCM should be matched with outdoor meteorological parameters to avoid excessive use of PCM (will make increasing costs and deteriorating indoor light environment) or inadequate amount of PCM (PCM will melts prematurely and does not conducive to indoor thermal environment).