Abstract: For choosing right gyro and accelerometer to design a good strapdown inertial navigation system(SINS) according to its precision need，a new error analysis method combining theory with engineer design experience was proposed. The functions of inertial sensor-error,velocity-position error and attitude error under difference working conditions of SINS were set up.One method that for designing a SINS with the position-error less than 100 m during 2 min was suggested.