Quantum error mitigation is crucial for us to protect quantum computing against quantum errors before quantum error correction is truly available, which is still one or two decades away. Though some error mitigation protocols, like error extrapolation and error cancellation, have been demonstrated successfully in experiments using small scale quantum systems, whether they behave well on large scale quantum computers remains unclear. Recently, it has been found out that after error mitigation, the remaining error is roughly of order the square root of N, where N is the number of quantum gate number.