The invention discloses a detergent for cleaning underwear and a preparation method thereof. The preparation method comprises the following steps of 1), dissolving triclosan in C9-11 alkanol polyether-3 and coco trimethyl ammonium chloride to obtain a mixed solution, 2), heating disinfected water to a temperature of 85 to 90 DEG C, adding sodium laureth sulfate into the heated disinfected water, carrying out heat preservation and stirring for dissolution, 3), cooling the solution obtained by the step 2 to a temperature of 68 to 72 DEG C, adding laureth-9 and laureth-7 into the solution, carrying out heat preservation, stirring for dissolution, adding borax and anhydrous calcium chloride into the solution and stirring for dissolution, 4), cooling the solution obtained by the step 3 to a temperature of 58 to 62 DEG C, adding C14-17 sodium secondary alkane sulfonate into the solution, and stirring for dissolution, 5), cooling the solution obtained by the step 4 to a temperature of 49 to 52 DEG C, adding the mixed solution obtained by the step 1 into the solution obtained by the step 4, and stirring for dissolution, 6), cooling the solution obtained by the step 5 to a temperature of 46 to 48 DEG C, adding sodium chloride into the solution, carrying out adjustment and uniformly stirring, and 7), cooling the solution obtained by the step 6 to a temperature of 40 to 45 DEG C, and adding alkali protease, methylchloroisothiazolinone, methylisothiazolinone and essence into the solution. The detergent for cleaning underwear can effectively remove blood stains, perspiration stains, milk stains and endocrine. The detergent for cleaning underwear does not contain components such as phosphates, aluminum and a fluorescent brightening agent harmful to the environment and the ecology.