This paper presents a 2MHz CMOS active rectifier for wireless power transfer (WPT) systems with PWM mode current compensation. Using PWM mode to generate compensation current adaptively, both on/off delay are eliminated. This rectifier achieves high power conversion efficiency (PCE) and voltage conversion ratio (VCR) under various loading and input power conditions. Moreover, with PWM mode, compensation current range can be wide and flexible. This active rectifier was designed in 0.18µm CMOS process. Simulation results show that, with input ac voltage from 2-4V, peak PCE achieves 92.6% with 100Ω loading, peak VCR achieves 98.4% with 1KΩ loading, and maximum output power reaches 112mW.