In the conventional works on neutral grounding modes for mediumvoltage distribution system, the single factor such as reliability is only considered in various grounding modes, which neglects the comprehensive effect of reliability, security and economy on grounding modes. Therefore, online support tensor machine classification algorithm is proposed to analyze the reliability, security and economy in threedimensional space. Specifically, the hierarchical analysis algorithm, fuzzy synthetic evaluation algorithm and fuzzy clustering method are proposed to establish the evaluation model of reliability, security and economy, respectively. The weight tensor is derived by solving the effective kernel function, then the optimal neutral grounding mode is obtained. Simulation results show that, compared with the above algorithms, the proposed algorithm could keep the structural information of data, and the data timeless is improved.