The harmonic distribution of a traction substation is determined by the traction load harmonics and the system background harmonics. The traction load is essentially random and fluctuating， so there is generally a certain proportion of no-load for a traction substation. Considering the probabilistic characteristics of harmonic voltage on the high-voltage bus of substations， a multi-parameter probability distribution function is proposed to describe the distribution of the superposition of background harmonic voltages and traction load harmonic voltages. Using the simulated annealing algorithm， a curve fitting approach is put forward， which can be used to identify the parameters and then to confirm the harmonic emission levels of traction loads. Finally， the feasibility and practicability of the proposed method are validated by actual case calculations.