Based on orthogonal wavelet series decomposition, there is often inaccuracy or huge amounts of calculation when identifying the load due to improper choice of wavelet series order. The envelope line of the wavelet series coefficient amplitude is proposed, which tends to converge with the increasing order through the matrix spectral decomposition and Parseval theorem of which represents that the total energy in the time-domain and frequency-domain is equivalent. The non-analytical wavelet coefficient is analyzed, and the function between the wavelet series order and the relative error of the identified load is obtained. The wavelet series order is thus chosen according to the error level of the engineering requirements. The theoretical basis of the order selection when using the series decomposition in the dynamic load to be identified is proposed. Both the computer simulation in the form of the single-frequency and multi-frequency, real signal as well as the lab test in the form of the impact signal confirm that the proposed approach is feasible and effective. The results show that the proposed approach is applicable to all forms of the loads.