Modifying the expansion characteristics of mudstone filler is a feasible way to reduce the arch deformation on mudstone roadbeds. Through the free expansion rate test, no-load and loaded expansion rate test on the modified mudstone filler with NaCl solution, the response rule between the expansion rate of mudstone filler and the concentration of NaCl solution and the mixing ratio of NaCl solution was investigated, and the microstructure of mudstone filler before and after the modification was analyzed by nuclear magnetic resonance (NMR) test. The results show that: (1) The modified mudstone filler with NaCl solution concentration of 12% and doping ratio of 14%, the unloaded expansion rate and loaded expansion rate decreased by 10.651% and 75.439%, respectively, compared with that of the unimproved mudstone filler; (2) With the increase of NaCl solution concentration, the percentage of small pores of mudstone filler showed a tendency of increasing and then decreasing; (3) Under the same NaCl solution concentration, the percentage of small pores of mudstone filler also showed a tendency of increasing and then decreasing with the increase of NaCl solution mixing ratio; (4) NaCl solution concentration and small pores of mudstone filler jointly affect the intergranular stress of mudstone filler and thus change its expansion.