The invention discloses a combined cycle unit effect enhancing control system and control method based on SCR (semiconductor control rectifier). The control system comprises parameter detectors arranged at detection points and used for detecting corresponding detection parameters and a controller. The optimal combustion chamber outlet temperature is determined according to a rational/semi-rationalformula, the various detection parameters and achievable maximal denitration efficiency; whether the optimal combustion chamber outlet temperature is within a safe threshold value of a fuel gas turbine or not is judged, if the optimal combustion chamber outlet temperature is within the safe threshold value, the achievable maximal denitration efficiency can be determined to be target denitration efficiency, and the fuel gas turbine executes the optimal combustion chamber outlet temperature; otherwise, the achievable maximal denitration efficiency is revised till the corresponding optimal combustion chamber outlet temperature is within the safe threshold value of the fuel gas turbine. The combustion chamber outlet temperature is increased to the largest extent by adjusting the denitration efficiency under the situation that emission of the nitric oxide concentration meets the standard requirement and the fuel gas turbine operates in the safe range, and accordingly the power generating efficiency of the fuel gas-steam combined cycle unit is improved.