Variable optical attenuator（VOA）is a key power controlling device which can realize channel equalization and automatic gain control（AGC）. Owing to common characteristic of wide dynamic range

low power consumption

compactness and convenience of integration

micro-electromechanical system（MEMS） and thermooptic are important techniques for VOA. Polymer with optimum thermooptic coefficient became the most attractive thermooptic materials. We classified the presented VOA firstly. Various MEMS-type VOA were introduced. We also summarized the developments of polymer

and from the angle of thermooptic material

emphasized on the analysis of various polymer-based thermooptic VOA. Two application instances in optical network were presented lastly.