Abstract: To improve the adverse effects of the depletion of plant gum resources on the application of flushing fluid in complex strata such as coal-bearing strata, a study was conducted on the compound and crosslinking modification of guar gum. A new type of plant gum flushing fluid (GXB) was developed using xanthan gum as the compound synergist and organic boron as the crosslinking modifier, with cross-linking control agent and inorganic treatment agent as the supplementary. Meanwhile, a series of performance evaluation was carried out. The evaluation results show that GXB has excellent performance and can meet the requirements of drilling in complex strata such as the coal-bearing strata. In addition, the mechanism of its compound-crosslinking modification was analyzed with the infrared spectrum analysis, viscosity analysis and nuclear magnetic resonance analysis in combination. The results show that: (1) There are two mechanisms of action in the process of compound synergism. Firstly, the galactose-free regions in guar gum could form hydrogen bonds with xanthan gum molecules or van der Waals forces to form a three-dimensional network structure to increase the viscosity. Secondly, the xanthan gum is prone to self-association under the action of guar gum to improve the structural viscosity of the system. (2) The viscosity of the system is further improved by the cross-linking reaction of borate ions in organic boron by complexing the hydroxyl group on galactose. The process can be divided into four stages: the initial stage, the transitional stage, the ascending stage and the stable stage. Besides, the speed of the cross-linking reaction in each stage is mainly affected by the release rate of borate ions by the organic ligand. Generally, the research results provide a new solution to solve the problem of coal-bearing strata collapse and have a certain reference value for the study on modification of plant gum flushing fluid.