Focusing on the problems such as low track coverage and weak adaptability to the irregular flow field in the current symmetrical track model of transit-time ultrasonic flow-meter，the design theory of 3D multi-path model is explored． Firstly，the formula for calculating the minimum number of paths satisfying the expected coverage is deduced． The distance between each path and the center of the pipeline is solved． The number of paths and the number of 2D track model are calculated． The angle between adjacent path segments of a 2D track model of a single path is calculated． Then，the coordinates of multi-path topology node are solved． Finally，multi-path topology is designed． The example calculation and application show that the design method of transit-time ultrasonic flow-meter 3D multi-path model is feasible，advanced and practicable．