Directional coupler can be constructed by parallelly putting five photonic crystal waveguides together. The propagation of the optical field entering this system symmetrically is analyzed numerically on the basis of self-imaging principle. 1×3 beam splitter,whose length can be as short as 14.26 μm,is designed by using this structure. By symmetrically tuning the effective refractive index of two dielectric rods in the coupler,and accordingly inducing the redistribution of the electric field,equipartition or free distribution of output energy can be achieved. By asymmetrically tuning the effective refractive index of one dielectric rod in the coupler,free distribution of output energy can also be achieved. Due to the ultracompact size and the free modulation of output energy,the device has extensive practical value in future photonic integrated circuits.