The in-bore magnetic field distribution characteristics of electromagnetic launcher should be considered in the layout design of components in guided projectile. The high in-bore magnetic field generated by electromagnetic launcher is analyzed, and a formula for magnetic field distribution is deduced based on Biot-Savart law, in which projectile displacement and current skin depth are considered. The relationship between inductance gradient, current frequency and projectile＇ s location is obtained by timeharmonic analysis and data fitting to analyze the in-bore kinetic characteristics of projectile interior trajectory. And the time-frequency analysis method is used to achieve the relationship between current skin depth and time to build a 3-D theoretical calculation model （TCM） for the distribution characteristics of magnetic field along the central axis of projectile. A laboratory electromagnetic launcher is taken for example, and the experimental data is used as input for simulation. The results show that the in-bore magnetic induction intensity frequency is below 450 Hz and the peak of in-bore magnetic induction intensity is up to 0. 4 T, which gets weaker along the length direction of projectile and approximates to 0 at 100 mm. The validity of TCM is verified by using the experimental data of magnetic probe.