Power is one of the basic parameters of microwave signal. Traditional power measurement methods are difficult to adapt to distributed long-distance network applications due to high transmission loss and large volume of coaxial cables. In this paper, a method of microwave power measurement based on electro-optic conversion is proposed. The microwave power measurement was converted to the measurement of optical sideband rejection ratio. The mapping relationship between microwave power and optical sideband rejection ratio was analyzed respectively for electro-optic conversion methods using phase modulation and intensity modulation. Finally, the compliance between the measured value and the theoretical value was compared. After electro-optic conversion, this method can use optical fiber to transmit signals and carry out long-distance testing, which has a great application prospect in distributed long-distance networking.