The invention provides a method for preparing carboxymethyl starch sodium by the microwave non-alcohol solid phase one step alkalizing method, which makes creative improvements based on the prior dry-process production technique. The method uses micro waves to heat, thereby the reaction efficiency is high, the reaction time is short and the production efficiency is high; moreover, no dispersant is used, thereby while microwave heating, the charring of starches due to overheat does not happen, and the reaction temperature is easier to control; the alkali and chloroactic acid are both respectively added once, thereby simplifying the technological process; organic solvents such as ethanol, etc. are not used in the reaction process, thereby lowering production costs.