The invention discloses a square ring filter which includes an upper component, a middle component, a lower component, an input port, output ports, waveguides and metal films. The output ports includea first output port, a second output port and a third output port. The waveguides include a first waveguide, a second waveguide, a third waveguide, a fourth waveguide, a fifth waveguide and a sixth waveguide. The metal films include a first metal film and a second metal film. The input port and the first output port are located between the upper component and the middle component. The first metalfilm is located between the first waveguide and the second waveguide, and an obstacle is set in the second waveguide. The second output port and the third output port are located between the lower component and the middle component. The second metal film is arranged between the fifth waveguide and the fourth waveguide. By setting an obstacle in the second waveguide, light can go out from different output ports. The ratio of intensity of light going out from different output ports can be controlled. The energy of the three output ports is analyzed. Therefore, the functions of filtering and beam splitting are achieved.