Abstract: This paper researches how to establish the load spectrum for motorized spindle reliability test,which ba-ses on investigating and collecting the milling technology and data of the domestic 170XD30 milling motorized spin-dle. Firstly,the sample data of the motorized spindle's load spectrum was sorted out after analyzing and calculatingthe relative milling data. In addition,the frequency distribution point map and cumulative frequency distributionpoint map of the milling motorized spindle's load were drawn and the overall distribution model and parameter esti-mation were initially determined. Finally,it conducted a test of distribution fit by using K-S test method,builtedthe distribution model and the five-test load spectrum of the milling motorized spindle.