This paper presents a multi-mode boost converter to achieve the high efficiency over a wide input voltage range for thermoelectric energy harvesting (TEH) applications. Under high, medium, and low input voltages, continuous conduction mode (CCM), critical conduction mode (CRM), and discontinuous conduction mode (DCM) are adopted to improve the conversion efficiency, respectively. Moreover, a constant peak current (CPC) control scheme is proposed to realize high efficiency under ultra-low input voltage. The proposed converter is implemented in a $0.18 \mu \text{m}$ standard CMOS process, which occupies a chip core area of about 1.0 mm $\times0.9$ mm. It achieves >60% efficiency in the range of TEG open-circuit voltage $V_{S}$ from 20 mV to 500 mV, and a peak efficiency of 91.4%. Moreover, after the startup is completed, an efficiency of 37.1% under the $V_{S}$ as low as 10 mV can be achieved.