In this paper, formulae of calculating best operating position of horizontal pendulum tiltmeter are derived, choice of precision and para-meter are discussed. After horizontal pendulum tiltmeter is fast and precisely adjusted by using this method given in this paper, tiltmeter operates in best position. Deflection angles of true operating position of instruments from their best positions, which were installed at more than 50 stations in China, were determined by using this method, and these instruments were readjusted, better effect was taken.