Abstract: A microwave high-speed QPSK modulation circuit is designed in this paper. Ring modulator architecture is used in the main circuit, and it′s composed by ring quad Schottky diodes and microwave wide-band Balun. The modulation date rate can reach 500 Mbit/s in X band by using high speed emitter-couple logic (ECL) difference circuit, while the error vector magnitude (EVM) is about 5%. The circuit has a high performance by optimizing circuit and adding design of compensating circuit. After testing sample in X band, the conversion loss is less than 12 dB, the quadrature state amplitude and phase consistencies are less than ±0.25 dB and ±2°, and carrier suppression is more than -38 dBc. This circuit design have many advantages, including the simple and compact structure, high precision, high reliability, and this design provides reference to engineer application for microwave high-speed modulation circuit.