Integrating solar thermal energy into conventional coal-fired power plant has been proved to be an efficient way to use solar energy and improve the generation efficiency of coal-fired power plant. Because of the variability of solar irradiance, an optimization strategy based on switching solar integrate mode according to the solar irradiance is proposed in this paper. A parabolic trough solar aided coal-fired combined heat and power (SACF-CHP) system model is built to analyze the system performance. In order to obtain the minimum Levelized Electricity Costs (LEC), a generic superstructure of solar integrate mode is established, and a mixed integer nonlinear programming approach is applied to optimize the oil-water heat exchanger area and solar integrate mode. The optimized results of a 150 MW SACF-CHP system indicate that switching integrate mode according to the solar irradiance would have a lower LEC than fixing the integrate mode invariably, and the optimal LEC of those two integrate modes are 0.0771 $/kW h and 0.0794 $/kW h respectively. Besides, the LEC decreases at first and then increases with the increase of the oil-water heat exchanger area. And the optimal heat exchanger area for changeable integrate mode are 725 m(2). (C) 2017 Elsevier Ltd. All rights reserved.