Monolayer WSe2 exhibits unique optical and electronic properties, showing great potential applications in functional integrated devices, such as electronic devices and optoelectronics. Understanding the growth behavior and process are the key points for the salt-assisted growth of large domain WSe2 monolayers, it is also very important for its further application in on-chip laser and opto-devices. Here, we report a NaCl-assistant method for controlled growth of single crystal monolayer WSe2 with a domain size up to 0.57 mm on SiO2/Si substrate. Atomic-resolution scanning transmission electron microscopy reveals that the Se1 and Se2 vacancy point defects are the main defect type of those materials. The growth behavior of the salt-assisted method have been systemly investigated. The loading mass of NaCl powder prefers to be less with the controllable vapor process. The flow of hydrogen gas was also preferred to be suitable with a weak etching effect. The morphology of monolayer WSe2 shows a sensitive temperature dependence evolution with the growth temperature increasing. A screw dislocation growth behavior with 15° angle is also observed with the NaCl-assistant method. The results provide a deep understanding of the mechanism for the NaCl-assistant growth of large size monolayer WSe2.