A measurement method for micro topography and roughness of internal surface of gap belongs to the technical field of precision measurement and is realized based on a measurement system which comprises a PC, a controller, a flexible mechanism and a measurement thin film. The measurement thin film has a copy function and is bonded to the flexible mechanism. The PC is connected with the flexible mechanism through the controller to control the flexible mechanism to expand or contract. The measurement method can effectively solve the measurement problem of the micro topography and surface roughness of the internal surface of the gap with a narrow inlet size. The method is simple and easy to operate, and the device is easy to carry, low in cost and high in measurement accuracy.