Abstract: Karst is common in the area where the Guizhou-Zhangzhou-Changzhou railway passes through, of which landforms are characterized by depressions, sinkholes, caves and cracks. The roadbeds in this area are mostly susceptible to collapse as shown by regional geological evaluation. The consistency of evaluation of grouting effects is a difficult issue. At present, there are many methods to evaluate the effect of karst treatment, which, however, are mainly qualitative. This work aims to explore a quantitative method by pressure water tests to evaluate the grouting effect in karst areas. Firstly, various evaluation methods are presented. Then feasibility of quantitative evaluation of grouting by pressure water tests is analyzed. A representative section of grouting tests at the karst roadbed of the Qianjiang-Changde railway is selected as an example and the different cases with four kinds of qualified criteria are examined, indicating that the current qualification criteria have some advantages and disadvantages. Combined with theoretical analysis and practical application, a new criterion for quantitative evaluation of the grouting effect is established on the basis of many tests and optimization schemes. And the technological parameters of the pressure water test are summarized. The pressure water test method used in this work is a single-point method, in which the number of the pressure water tests before and after grouting is no less than 5% and 4%, respectively; the test pressure is 0.05-0.25 MPa, the test length is 3-6 m, and the pressure water time is 5-10 minutes per time. The results show that the new evaluation criterion verified by 44 sections of water pressure tests is more reasonable and accurate than the other single criteria. Meanwhile, the fuzzy evaluation of drilling, and geophysical prospecting is used to verify the criterion again, which suggests that the criterion is basically consistent with other evaluation results, showing good applicability and rationality. In all, this criterion is a suitable and reliable method for quantitative evaluation the grouting effect on the karst roadbed.