The focusing accuracy of the excitation laser on the target surface has a great influence on the stability of the spectral signal in the laser-induced breakdown spectroscopy (LIBS) system. But it is difficult to pretreat the natural samples in some special application scenarios, such as the LIBS detection of sediments in the vicinity of deep-sea hydrothermal vents, to improve the focusing accuracy. It is significant to design a focusing system with fast speed and high precision for in-situ LIBS detection. In this research, the microscopic autofocus system combined with the image contrast evaluation method is applied to the LIBS detection system in order to improve the focusing accuracy, and the two-color laser diode assisted focusing method is used to expand the focusing range and accelerate the focusing speed. The experiments show that combining the two focusing methods can realize 2400 μm focusing range and 20 μm focusing accuracy, and can speed up the focusing process. The system proposed is hopeful to improve the LIBS signal quality and spectral acquisition efficiency in field detections.