To prolong the treatment cycle of crystallization blockage in tunnel drainage systems, reduce maintenance costs, and lower crystallization risk, novel materials and processes for preventing crystallization blockage are developed, and scale inhibitor-based crystallization prevention measures for tunnel drainage systems are proposed. Scale inhibitor monomers suitable for the tunnel application environment are screened through investigation and testing, and indoor performance tests of binary composite scale inhibitors are conducted. Furthermore, the synergistic effects of different composite scale inhibitors are  examined, the composite scale inhibitor capable of effectively slowing crystallization is identified, and phase analysis, microstructural characterization, and infrared spectroscopy are employed to investigate the synergistic mechanism of the optimal composite scale inhibitor. Finally, an application scheme is proposed, and the engineering performance of the composite scale inhibitor is verified through tunnel field tests. The results show the following: (1) The polyaspartic acid（PASP）∶ phosphonobutane-1,2,4-tricarboxylic acid(PBTCA) (1∶1) binary composite system exhibits the best performance, with a scale inhibition rate of 94%. (2) Microscopic analysis indicates that the PASP∶PBTCA composite scale inhibitor promotes the formation of unstable vaterite calcium carbonate, accounting for 60.3%. Based on multiple scale inhibition mechanisms, including chemical chelation, lattice distortion, physical dispersion, and surface adsorption, this composite scale inhibitor suppresses calcium carbonate crystal formation through the synergistic action of multifunctional groups. (3) Field test results confirm that the composite scale inhibitor effectively inhibits calcium carbonate crystallization adhesion on drainage pipe walls and exhibits good dispersion performance, thereby interrupting the vicious cycle of calcium carbonate crystallization, precipitation, adhesion, and growth.