Free time-of-flight expansion of ultracold atoms is an important method to obtain the information of atomic cloud in the momentum space. Magnetic Feshbach resonance manipulates the interaction between atoms, therefore it requires free time-of-flight expansion under high bias magnetic field to obtain the information of interaction energy. Therefore, the spatial distribution of magnetic field has an important influence on the free time-of-flight expansion of atoms.In this paper, we construct two kinds of magnetic coils, and calculate the magnetic field distribution under Helmholtz configuration and study the free time-of-flight expansion of sodium atoms Bose-Einstein condensate. The magnetic field coil parameters affecting the shape and size of atomic cloud after the free time-of-flight expansion are analyzed and discussed. This work provides an important suggestion for designing magnetic Feshbach resonance coils in the future.