Abstract: Wavelength division demultiplexer is an important component used for dropping optical signals in the design of optical communication systems.The characteristics of micro resonator used for dropping optical signals are closely related to the performances of wavelength division multiplexing(WDM) demutiplexing systems. Single channel WDM demultiplexing structure based on photonic crystal square resonator(PCSR)was gradually optimized in the two-dimensional photonic crystal. With the help of coupled-mode theory(CMT), electromagnetic wave coupling interactions between waveguide and cavity were analyzed qualitatively, and operational characteristics of the structure were numerically simulated by the finite-difference time-domain(FDTD)method.The study results show that the single output port WDM demutiplexing structure based on PCSR has the characteristics of single resonant peak, broadly tunable ranges of central wavelength(1 501.4 nm~1 591.0 nm), narrow passband width(3.3 nm~9.1 nm)in the ranges of design parameters. It can be used for designing photonic integrated circuits, WDM demultiplexing optical communication systems, and etc.