How to judge the searching precision of Particle Swarm Optimization?
If by "precision" you mean the error in the result of PSO (i.e., the difference between the value that PSO outputs and the true global optimum), the error can be arbitrarily large.  PSO is a heuristic.  There are no guarantees that it finds the global optimum, or even that it gets close -- and there is no formula in terms of parameters (like velocity) for how large the error will be.  The error depends on the optimization problem you're solving and on the shape of the loss surface.