A high linearity broadband IQ modulator circuit designed in a 0.1 μm GaAs PHEMT process is presented. It can support a baseband bandwidth of 0–2.5 GHz and RF bandwidth of 3–20 GHz. A three-stage cascaded passive polyphase filter (PPF) is used to generate four phase quadrature broadband signals in the local oscillator chain. Shunt peaking is adopted to expand the bandwidth of the RF driver amplifier and an active Gilbert mixer is designed to acquire a certain gain. The linearity of the modulator is improved by optimizing the amplitude of the local oscillator driving signal, and the modulator chip is applied to high performance measuring equipment. Circuit simulation results show that the output frequency covers 3–20 GHz, the in-band conversion gain is greater than 7.5 dB, OIP3 is greater than 21.8 dBm, OP1dB is greater than 12.7 dB, and the input/output return loss performance is excellent. The circuit measurement results show that the in-band conversion gain is no less than 5.7 dB, OIP3 is no less than 20.26 dBm, and the total power consumption of the chip is 3.4 W.