The invention belongs to the field of medicinal chemistry analysis and particularly relates to a method for detecting 5-Isoquinolinesulfonic acid in fasudil hydrochloride. The method is that a high-performance liquid chromatography-mass spectrum system is adopted to detect a 5-Isoquinolinesulfonic acid reference substance solution and a fasudil hydrochloride test sample solution. According to thedetecting method, the high performance liquid chromatography is firstly adopted for component separation, the position of a chromatographic peak of the 5-Isoquinolinesulfonic acid in the fasudil hydrochloride test sample solution can be determined under the chromatographic condition, the chromatographic peak of the 5-Isoquinolinesulfonic acid is effectively separated from chromatographic peak of adjacent other components, and the operation time is short; then, the mass spectrum condition is adopted to quantify a ternary quadrupole rod mass spectrum, ppm level of 5-Isoquinolinesulfonic acid canbe detected, and the limit of quantitation can reach ng/ml. Therefore, the 5-Isoquinolinesulfonic acid in fasudil hydrochloride can be effectively controlled.