The invention relates to a flow line type oil changing system and method for a wind power generation set. The system comprises a waste oil drum, a cleaning oil drum, a new oil drum, a steel cable fixer, a tensioner, a safety ring, a lifting hook ring, a lifting hook, a hook rope, a safety steel cable, an hoister and a tower. The method includes: waste oil extraction, alternating between waste oil extraction and cleaning, cleaning, alternating between cleaning and new oil conveying, and new oil conveying. The oil changing system and method has the advantages that by the system and method, oil changing can be performed conveniently, and a large amount of oil changing time and labor can be saved; the safety steel cable is used, so that safe oil changing is achieved; the system is practical in structure and high in safety; the method is novel in operation, low in difficulty, high in efficiency, suitable for oil changing of various high-altitude mechanical devices, and especially suitable for occasions that the devices requiring oil changing are located at relatively-high positions.