In the WDM optical network, a tunable F-P cavity is often used as a demultiplexer. But the traditional structure has the shortcoming that the reflective light is too strong and this will bring trouble to the transmmiter and EDFA in the network system. To solve this problem, isolators are often used. But this will cause an extra loss. Besides, it will costs too much. A new type of tunable F P cavity is designed in this paper. Its insert loss is low and there is almost no reflective light from the F-P cavity. Finally, it can be tuned electronically.