It is necessary to analyze the impact of various operation conditions on the harmonic of power grid in distributed generation (DG) access programing. In view of large calculation quantity deficiency of Monte-Carlo probabilistic harmonic flow method which need large sample quantity to ensure calculation accuracy, firstly, partial decoupling harmonic flow method is adopted to solve the deterministic harmonic flow, and voltage harmonics distortion rate close to the limit is chosen as kernel samples. Secondly, important sampling method is used to make Latin hypercube sampling have higher probability to focus on the concerned parameter region, thus the size of samples is reduced, then the validity and correctness of the proposed method in IEEE-30 node test system is verified. Finally, the impact of amplitude correlation and harmonic phase randomness of distribution generation on the distortion rate of harmonic voltage in IEEE 33-bus system is computed and analyzed, and it is necessary to consider the amplitude correlation and harmonic phase randomness when optimizing the access of DG.