The unpredictable propagation of ambient radio frequency (RF) waves may cause significant randomness in power densities against the propagation distance, time, and locations. To address this challenge, we propose a novel adaptive rectifier system with stabilized output dc power against dynamic power densities. A novel detection circuit is designed to identify the frequency range and receiving power level. Then the received RF power is amplified to the saturated level by referring to the detected frequency and power information for adaptive dc biasing control for an integrated power amplifier. Doing so makes the input power to the rectifier constant regardless of the power and frequency variations. The experimentally validated system can output a stabilized power under the time-varying power range of -10 to 0 dBm and location varying within 0.5 m at 1.8, 2.1, and 2.6 GHz. Compared with conventional designs, this system has realized stabilized and enhanced conversion efficiency (61% at 1.8 GHz, 64% at 2.1 GHz, and 53% at 2.6 GHz) and therefore could be adopted for wireless powering distributed sensors and Internet of Thing (IoT) devices continuously and steadily at flexible conditions.