The invention discloses an apparatus for large-scale oil-immersed transformer no-load and on-load comprehensive tests and a method. The apparatus is composed of a test power source part and a measurement system. The test power source part includes an induction regulator, an intermediate transformer, an oil-immersed intensive compensation capacitor bank and a test control system which are successively connected via a circuit. The measurement system includes a precision measurement current transformer, a voltage transformer and a power analyzer. The apparatus can conduct on-site no-load and on-load loss measurement tests on 110-kV and 220-kV voltage class transformers, and can complete measurements of no-load current, no-load loss, no-load harmonic waves, short circuit impedance and load loss of transformers, and can automatically calculate the measurement test results of no-load and load loss.