In order to solve the problem that the schlieren method cannot accurately measure the weak signal region of the sidelobe beam in large laser devices of the national large scientific facility， a new schlieren method based on the diffraction inversion of the sidelobe beam is proposed to measure the far-field focal spot for high-power laser. The key point of this method is that an indirect measurement approach is used based on reverse deduction， while deducting along the reverse direction of the optical path propagation. The diffraction intensity image and phase image of the sidelobe beam are the inputs to calculate the far-field focal spot distribution of the front sidelobe beam， which is not shielded. Compared with the traditional far-field focal spot measurement based on the schlieren method， the improvements and optimizations proposed in this paper are as follows. First， the mathematical model of far-field focal spot measurement using the schlieren method is improved to reveal the rationality of the model， theoretically based on the principle of diffraction inversion of the sidelobe beam and the indirect measurement approach. Then， the feasibility of this method is verified by simulating the whole experimental process of high power laser far-field focal spot measurement， which consists of sidelobe beam diffraction， denoising， sidelobe beam diffraction inversion， and focal spot reconstruction. Finally， the improved DnCNN algorithm is used to remove the noise of different levels （0-75 dB） of 12 bit scientific CCD images in the mainlobe and sidelobe beams， and the reconstruction accuracy of far-field focal spot is improved. The experimental results show that this method not only eliminates the influence of the schlieren sphere on the diffraction of the sidelobe beam but also obtains the real intensity distribution of the sidelobe beam in the weak signal region， including the important parameters of far-field focal spot measurement， such as the amplitude and position of each peak of the side lobe beam， the dynamic range ratio， the amplitude and position of each peak of the sidelobe beam， and the ratio of dynamic range. The error between the reconstructed and theoretical focal spots of the dynamic range ratio is 3.20%. It is significant to improve the reliability and experimental accuracy of the far-field focal spot of high-power laser measurement using the schlieren method.