Range of Variability Approach (RVA) is an important method to evaluate the alteration of river streamflow. However, the traditional range of variability approach (RVA) has the drawback that variabilities of low and medium flow indicators are often ignored. To assess flow regime alterations more comprehensively, an improved RVA method was proposed based on the CRITIC method (Criteria Importance Through Intercriteria Correlation) combined with Euclidean distance. The observed streamflow data of the Tangnaihai station from 1960 to 2009 in the source region of the Yellow River were used to validate the rationality of the new method. Meanwhile, the change trend of hydrology index and the overall degree of hydrological variation in the region were analyzed. The results show that the annual streamflow indicate a decreasing trend after the mutation in 1990. After the disturbance, the overall changing degrees calculated by the improved method is 27.33%, belongs to medium change. Nine IHA indicators, including mean flow in February/March, annual minimum 7/30/90-day flow, annual maximum 90-day flow, mean duration of high flow, decreasing rate and increasing rate of flow, belong to moderate change, while the rest of the indicators belong to low change. Compared with the traditional RVA, the improved RVA method can effectively consider the impact of low and medium indices and make the evaluation results more objective.