The defocusing effect of computational ghost imaging (CGI) through atmospheric turbulence is investigated, and the incoherent Lorentz source is proposed to suppress the defocusing effect. The imaging formula is derived to describe the defocusing effect of CGI in the atmosphere, and the comprehensive effects of atmospheric turbulence and defocusing on the imaging resolution of CGI with an incoherent Lorentz source are studied, respectively. It is found that the imaging quality of CGI under the non-defocusing condition is better than that under the defocusing condition in free space. However, for long-distance CGI in atmospheric turbulence, considering the comprehensive effects of defocusing and turbulence, the negative defocusing effect achieves high-quality imaging by shortening the effective propagation path and suppressing turbulence perturbations, which can be exploited in practice. Furthermore, for long-distance propagation, the incoherent Lorentz source can suppress the defocusing effect over a wider range of defocusing lengths compared to the Gaussian source with the same beam size.