The work aims to explore the feasibility of screening the moisture-proof inclusion agent with mono-ammonium phosphate powder by contact angle, through the study on the relationship between the contact angle and the moisture-proof effect with mono-ammonium phosphate powder coated with the organic reagent. Oleic acid, glycerol monostearate, lauryl amine, paraffin wax, stearic acid and cetyl alcohol were used as the wrapping agent of mono-ammonium phosphate powder. Washburn dynamic method was used to measure the contact angle. The moisture-proof effect was indirectly characterized by the moisture absorption rate. It was found that the moisture absorption rate of the coated powder would be decreased with the increase in its contact angle. The wrapping agent can be preferably selected by testing the size of the contact angle of the coated powder to modify the powder surface and improve the moisture-proof effect of the powder.