A preparation method of glyphosate comprises the following steps: introducing oxygen-containing gas into a mixture of N-(Phosphonomethyl) iminodiacetic acid with the weight concentration of 30-50%, water and activated carbon under a stirring condition for reaction with the pressure controlled at 0.1-1MP and the temperature at 30-90 DEG C, directly adding acid solution to reaction solution after the reaction to dissolve the generated glyphosate and directly separating out the active carbon, then lowering the temperature to precipitate the glyphosate, and filtering and drying to obtain the glyphosate. Filtrate can be reused as an acid material, the active carbon is washed with water for reuse, and washing water can be combined with the filtrate for reuse. The preparation method is characterized by simple process, low energy consumption, effectively reduced production cost and the like.