A wake-up receiver with an ultra-low-power envelope detector (ED) front structure is designed to address the sensitivity-power compromise problem of wireless receivers. Based on the analysis of the sensitivity constraint mechanism of this wake-up receiver, a passive differential envelope detector with a high input impedance is introduced to replace the traditional active envelope detector, and its design is optimized to reduce the power consumption of the system and improve the conversion gain and reception sensitivity. Designed using the 65-nm CMOS process, simulation results show that, driven by a 0.4 V supply voltage, on–off keying (OOK) signal with carrier frequency of 434 MHz, and data rate of 200 bits/s, the wake-up receiver consumes 16 nW and has a sensitivity of −68 dBm, which, compared with other devices, achieves a 44.7% sensitivity improvement with comparable power consumption. The 44.7% improvement in the sensitivity with the figure-of-merit (FoM) value is comparable to a 43.3% reduction in power consumption.