We describe an improved synthetic wavelength method for high-precision long-distance measurement with a repetition-rate-locked femtosecond laser modulated by a fiber Mach–Zehnder electro-optic intensity modulator. Harmonics of the repetition rate accompanied with modulating sidebands will be generated via intermode beating, which will be utilized for high-precision ranging. The nonambiguity range is significantly extended with a relatively low modulation frequency, and the ambiguous distance is unwrapped by synchronous phase-shift measurements of a synthetic wavelength chain without any auxiliary measurement operation. Our experiment shows a precision better than 20 µm at 46 m range, and a high-precision translation stage is applied for preliminary test and proof-of-principle demonstration. The demonstrated system is simple and can be easily integrated, and it will find widespread applications in large-scale metrology such as large-volume manufacturing and precision formation flying. © 2020 Optical Society of America