A liquid trap cup and a liquid trap assembly using the same are provided. The liquid trap cup comprises a hollow cup tube with a upper portion and a lower portion, a piston element with a top portion and a tail portion and a liquid discharge structure, in which the piston element is arranged airtight in the lower portion and is movable between a start point and an end point of the lower portion so as to control the opening and the closing of the liquid discharge structure. The liquid trap assembly includes the liquid trap cup and a multi-necked adapter tube with an expanded-plate structure, in which expanded-plate structure can be tightly sealed with the liquid trap. The liquid trap assembly is useful to a breathing system.