Based on Newton's second law and impulse theorem, the forces applied on the transportation piping by the fluid in the piping while the fluid flows have been analyzed according to the law of motion of transportation piping and flowing condition of the fluid. Through the theoretical analyses, the expressions of the forces applied on the piping by inner fluid while deep-sea mining system operates in general condition have been gained, and the physical meanings and calculating methods of the terms of the expressions have been discussed. The derived expressions can be directly applied into the force analyses of transporting system for deep-sea mining. If mining system operates in special conditions, the expressions also can be applied into the force analyses of the transporting system after hey are rationally simplified according to the specific operating conditions of mining system.