A new kind of low cost trajectory detection technology based on geomagnetic and acceleration meter is presented in this paper. The technology is that geomagnetic sensor cuts the magnetic-curves during projectiles rotating and produces electromotive force, calculates projectiles angular velocity of gun muzzle, calculates muzzle velocity through the correspondence between angular velocity and speed, provides initial value for the calculation of speed and position for the acceleration meter. As a new kind of aerial initial alignment, the technology provides a new idea for the application of inertia device as acceleration meter on the weapons with pipes.