The frequency doubling process in crystal has been reconsidered based on the theory of partial coherence in this paper. Two point P2 and P1 in the exit plane of the crystal is defined by the incident fundamental wave passing through lattice of which the primitive vectors are a and b respectively. Then the mutual coherence function of the light vibration at this two points at the frequency 2ω0 is calculated with the transformation technique in the frequency domain. Through introducing the definition p(ω) and its solution, the theoretical description of distribution of radiant intensity of the second harmonic wave excited by the incident fundamental wave based on the theory of partial coherence is finally obtained.