Thermo-optic modulators are key components of optical communication systems, and their performance directly affects system efficiency. With the development of silicon optothermonic technology, silicon thermo-optic modulators have been widely used in optothermonic chips. Conventional silicon optical modulators are large in size and have high losses. In recent years, researchers have proposed to use the slow light effect of photonic crystals to reduce the footprint of modulators. Related studies have shown that these devices have advantages, such as small size and low driving voltage. However, the optical transmittance of thermo-optic modulators based on photonic crystals is still affected by defects caused by fabrication errors. Valley photonic crystal optical waveguides can achieve scattering-immune high-efficiency unidirectional transmission, providing a new venue for realizing high-performance photonic devices. In this paper, a new silicon thermo-optic modulator based on a valley photonic crystal Mach-Zehnder interferometer (MZI) is designed. The electrical heating mechanism is introduced on one of the waveguides of the MZI. The thermo-optic effect modulates the refractive index to achieve precise phase modulation of the transmitted light. The thermo-optic modulator device has a small footprint of only 9.26 μm × 7.99 μm, which can achieve a high forward transmittance of 0.91, an insertion loss of 0.41 dB, and a modulation contrast of 11.75 dB. It can also be experimentally fabricated using complementary metal oxide semiconductor (CMOS) technology, so it will have broad application prospects. This modulation principle can be widely used in designing different thermo-optic modulation devices.