WCDA is a main component of LHAASO experiment, to measure the primary energy spectrum of gamma ray source is its main topic. The unfolding method is a traditional way in the study for the weak energy reconstructed ability of the ground array. In this paper, firstly, we estimate the significance of gamma ray signal from Mrk421 in one year. Then based on the minimum $\chi^{2}$ method, we study the detected ability of the spectrum of Mrk421 of WCDA at different observation times.