In order to study the probability power flow calculation of multiple power plants with output correlation.A combined probabilistic power flow calculation method based on Box-Cox transform and nonparametric kernel density model is proposed for multiple photovoltaic power plants.Firstly, combining Gumbel Copula function and Gini coefficient, the density function is established to describe the correlation and intensity of photovoltaic output.Then, based on Box-Cox normal transformation method and improved LDU triangulation method, the PV output is normalized and independent respectively,and comprehensive modeling of PV output is carried out. Finally, the Cumulant Method combined with Cornish-Fisher series expansion is used to calculate the probability power flow, and the probability distribution of node voltage and branch power flow is obtained and the results are compared with traditional nonparametric kernel density estimation and Monte Carlo method.The simulation results of a practical example show that the proposed method has high accuracy and less time，it took just 3.17 seconds，it has certain practicability and solves the problem of low efficiency of the traditional Monte Carlo method.