The invention provides a defect diagnosis method and device for a photovoltaic power station. The method includes the following steps: collecting test data during the operation of a photovoltaic powerstation to be diagnosed; according to the test data, determining an annual performance PR value of the photovoltaic power station to be diagnosed, a PR value of each power generation unit of the photovoltaic power station to be diagnosed, and the PR value, weighting efficiency, parallel mismatch loss or series mismatch loss of components included in each power generation unit; and diagnosing positioning defects of the photovoltaic power station to be diagnosed layer by layer based on a PR evaluation threshold model and according to the PR value of the photovoltaic power station to be diagnosed, the PR value of each power generation unit of the photovoltaic power station to be diagnosed, and the PR value, weighting efficiency, parallel mismatch loss or series mismatch loss of the components included in each power generation unit, wherein the PR evaluation threshold model is predetermined based on actual measured data of the photovoltaic power station on the scene. According to the above technical scheme, the defect detection of the photovoltaic power station from the operation level can be implemented, and the efficiency of defect diagnosis of the photovoltaic power station can beimproved.