In order to solve mixed problems of colored filaments in 3D printing, such as non-uniformed mixture and obvious edge of different colors, a novel method was presented to calculate the mixing chamber size based on the extrusion volume of filaments. Furthermore, the mixing chamber structure was analyzed from the structure, pressure and velocity distribution etc. The analysis results show that incidences of each filament and distributions of branch pipes have great influence on color mixture. The test results show that the method can not only realize mixture of multiple filaments, but also improve efficiency and provides a feasible method for 3D printing of mixing.