A novel heterogeneous solid base catalyst was prepared by loading of Ca-Al-graphite oxide with mixed potassium salts and applied in the transesterification of soybean oil with methanol to produce biodiesel. The catalysts were characterized by Hammett indicators, X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray spectrometry, and transmission electron microscopy. The effects of the methanol-to-oil molar ratio, catalyst amount, reaction temperature, stirring rate, and reaction time were investigated to optimize the transesterification reaction conditions. Moreover, the prepared catalyst retains its activity after being used for four cycles. In particular, the solid base catalyst can be effectively and easily separated from the reaction system, which may provide significant benefits for the development of an environmentally benign and continuous process for preparing biodiesel.