The invention discloses a method and device for determining a pipeline cleaning period of a long-distance liquid pipeline and belongs to the field of petroleum and gas. The method comprises the following steps: step a, determining a temperature range of a pipeline medium, and acquiring a minimum temperature T1 and a maximum temperature T2 within the temperature range; step b, determining a temperature T3 of a most seriously corroded pipeline segment medium in the pipeline; step c, making a corrosion immersion experiment on samples with the same pipeline material at T1, T2 and T3 respectively to obtain fastest pitting initiation time t0 of the samples at T1, T2 and T3; step d, performing indoor simulated pipeline cleaning operations with different periods on the samples at T1, T2 and T3 respectively; step e, performing a pitting rate test on the samples on which the step d is accomplished at T1, T2 and T3 respectively, and acquiring maximum pipeline cleaning operation cycles t1, t2 andt3, corresponding to the samples with the intermediate or below pitting rates, at T1, T2 and T3; and step f, taking a minimum value t4 of t1, t2 and t3 and determining an appropriate pipeline cleaningcycle of the pipeline as t0-t4.