The invention relates to a method for recycling waste steam turbine oil. The method comprises the following steps of: (1) a flocculation step: adding poly (diallyldimethylammonium chloride) with weight accounting for 10%-40% of the weight of oil to the steam turbine oil, stirring for 3-8 hours under the condition of constant temperature of 60-120 DEG C at the stirring speed of 100-300 r/min, and statically precipitating for 3-6 hours to complete the flocculation precipitation of tiny impurities contained in the waste steam turbine oil; (2) a decoloring step: adding coal ash with the oil weight of 5%-20%, the specific area of 300-600 m<2>/kg, the SiO2 content of 40%-60% and the CaO content of 5%-20% to flocculated oil, stirring at constant temperature for 3-6 hours under the condition of constant temperature of 80-130 DEG C and under the condition of 80-220 r/min, and precipitating for 3-6 hours under the condition of constant temperature; (3) an impurity separation step: filtering and separating the decolored oil by using a plate and frame filter press to obtain recycled finished oil. The method for recycling the waste steam turbine oil, which is disclosed by the invention, has the advantages of low recycling cost, simplicity in operation, relatively high recycled oil yield and good quality.