The influence of spatial coherence of light source on interference is explored on the basis of Young′s double-slit experiment. Firstly, the spatial broadening problem of light source is equivalent to the problem of double-slit with width illuminated by parallel light. Then the double-slit is approximated to multiple secondary light source point arrays with discretization in which the distance of adjacent points is extremely tiny. An algorithm is deduced based on the principle of wave vector superposition. In this algorithm, each pair of secondary light source points in double-slit is designed as an interference unit. The interference light intensity for each unit at each observation spot of view plane is calculated, respectively. Then, the interference intensity of all the units is linearly superimposed, and the total light intensity of the observation point is calculated. Finally, we use Matlab to simulate the light intensity distribution when the double-slit is vertically illuminated by monochromatic parallel light, Gauss spectrum and rectangle spectrum with different slit widths, respectively. Comparison of simulated results shows that the influence of the double-slit broadening on stripe contrast is much greater than that of the spectrum broadening. The proposed algorithm simplifies computed process and raises program operating speed remarkably.