WTe2, as a member of Weyl semimetals, is a vital candidate for the development of broad-wavelength-range photodetectors. At present, the preparation of WTe2 films mainly depends on the chemical vapor deposition (CVD) method. However, the chemical reactivity between W and Te is low, and the controllable synthesis of large-sized layered WTe2 in a stoichiometric ratio is the main challenge for further research. Here, we propose a salt-assisted double-tube CVD method for the one-step preparation of high-quality and large-size WTe2 crystals with a monolayer and few layers. The thickness and lateral dimension of WTe2 crystals can be effectively tuned by the growth temperature and hydrogen concentration, and the dynamic growth mechanism is understood by the combination of surface reaction and mass transport. Furthermore, a high-performance photodetector based on WTe2 is fabricated, which has high responsivity of 118 mA W-1 (1550 nm) and 408 mA W-1 (2700 nm) at room temperature, indicating its great potential for application in infrared optoelectronic devices. The results provide a reference for the preparation of 2D materials by CVD and lay the foundation for the fabrication of next-generation optoelectronic devices with a wide-wavelength-range response.