The invention relates to a testing device for a blade pretightening force and a testing method for testing the pretightening force by means of the testing device. The testing device comprises an electric vibration platform which supplies vibrations to a blade, and a laser sensor which tests the amplitude of the blade. The electric vibration platform is provided with a clamp for fixing the blade. The electric vibration platform is provided with an acceleration sensor which tests platform surface amplitude of the electric vibration platform. The acceleration sensor and the laser sensor are electrically connected with a controller which controls the electric vibration platform. The clamp comprises a clamp body. The clamp body is provided with a mortise which penetrates through from one end to the other end and matches a blade tenon at the root of the blade, and a pushing trough which is communicated with the mortise and is used for placing a pushing block. One side of the clamp body is provided with a bolt hole which is perpendicular with and is communicated with the pushing trough. The bolt hole is internally provided with a bolt which supplies a pretightening force to the blade. According to the testing method and the testing device, the pretightening force is applied through the bolt, thereby simulating an eccentric force for fixing the blade and the clamp. Testing is performed in a vibration environment which is highly approximate to the real working condition of the blade, thereby obtaining true and effective testing data.