Surface tension plays a significant role in fluid simulation, especially small-scale fluid. In this paper, we present a novel surface tension formulation for smoothed particle hydrodynamics (SPH) to simulate interfacial fluid flow. The surface tension formulation is decomposed into three main processes: (1) volume-preserved mesh smoothing, (2) surface tension computation and (3) surface tension transfer. Firstly, we exploit a Lagrangian operator to smooth an initial three-dimensional discrete interfacial surface mesh generated from fluid particles; and then the surface mesh is scaled in a volume-preserved way and the center is translated to its original position to get a smoothed mesh. Secondly, surface tension strengths on the vertices of the interfacial surface mesh are computed according to the offsets from the original surface mesh to the smoothed mesh. Finally, we transfer the surface tension strengths from the mesh vertices onto their neighbor fluid particles in a conservative way. The proposed surface tension solver is simple and straightforward to be plugged into a standard SPH solver. Experimental results show that it is effective and efficient to produce realistic fluid simulations, especially for the phenomena with strong surface tension.