The invention discloses an SCR reactor inlet flue gas soft measurement method based on a least square method. The method comprises the steps of analyzing a coal-fired unit SCR system mechanism, and determining variables related to the flue gas amount of an SCR reactor; acquiring historical operation data under different load stable working conditions; determining an SCR reactor flue gas quantity mechanism model according to an SCR reaction mechanism; calculating to obtain smoke quantity data under different load stable working conditions according to the smoke quantity data; calculating the relationship between the smoke quantity and the total air quantity under different stable working conditions by adopting a least square method; obtaining the relation between the smoke quantity and thetotal air quantity under different working conditions through an interpolation method, and calculating the smoke quantity at the current moment according to the total air quantity at the current moment. According to the method disclosed by the invention, the flue gas flow at the inlet of an SCR reactor is inversely calculated by utilizing the operation data, so that the real-time flue gas flow canbe accurately calculated, the problem of inaccurate measurement of the flue gas flow is solved, and the calculation accuracy of the ammonia spraying demand of the SCR reactor and the automatic ammonia spraying input adjustment effect are improved.