The present invention provides a series-parallel mismatch loss on-line test method. The method includes the following steps that: step 1, an on-line test device for series-parallel photovoltaic assembly mismatch testing is established; step 2, the effective data of the photovoltaic mismatch testing are acquired; step 3, the effective data of the photovoltaic mismatch testing are verified, if the effective data of the photovoltaic mismatch testing pass the verification, the series-parallel mismatch loss rate of a photovoltaic assembly is calculated; step 4, if the effective data of the photovoltaic mismatch testing does not pass the verification, the method returns to step 2, the effective data of the photovoltaic mismatch testing are acquired again; and step 5, the calculation result of the series-parallel mismatch loss rate of the photovoltaic assembly is obtained, after a test period is terminated, the on-line test device is dismantled, and the normal power generation of a photovoltaic array is restored. With the series-parallel mismatch loss on-line test method of the invention adopted, complete synchronization of the electrical performance parameters of a tested photovoltaic assembly and photovoltaic meteorological data is completed, transformation error of the transformation of measured data to an STC condition for series-parallel loss calculation in traditional series-parallel mismatch loss offline testing can be avoided, and the efficient and reliable operation of a photovoltaic power plant can be ensured.