The invention discloses a preparation method of low-molecular-weight polyglutamic acid. The method comprises the following steps: carrying out degradation by using a 1-6% polyglutamic acid water solution as a reaction substrate to obtain a degradation product, wherein the pH value is 1-6, the temperature is 60-120 DEG C, and the holding time is 1-8 hours; and adding an alcohol agent into the degradation product until the alcohol content reaches 65-91%, generating a precipitate, and collecting and drying the precipitate, thereby obtaining the low-molecular-weight polyglutamic acid. The invention also discloses application of the low-molecular-weight polyglutamic acid prepared by the method in preparing moisturizing cosmetics. A one-step process is used for degrading polyglutamic acid to obtain the degradation product within the specific molecular weight range without ultrafiltration or any other complicated after-treatment technique, thereby implementing complete utilization of the degradation product. The degradation product has excellent moisturizing property and lower viscosity, can be easily spread on the skin surface, reduces the risk of blocking the sweat pores and sweat gland pores, and is suitable for development and utilization of moisturizing cosmetics.