Our pendulum disk is made of natural marble. The suspended wire is made of isoelastic alloy which has been chosen through repeated testing for it's high quality factor, low internal exhaustion and almost zero thermoelastic coefficisnt. A low-damping free oscillating pendulum is set up instead of the discontinuous pendulum which is electro-magnetically attracted against a stop once every swing. Thus artificial disturbance to the pendulum is reduced. The pendulum is put in a vacuum-tight chamber to avoid environmental disturbance and lengthen the damping time. Besides, a multi-faceted reflective mirror ring is located at the centre of the pendulum disk. As a result, the number of times of measurement is increased from one to four or more within a certain period. This makes it possible to numerically solve the damping oscillating equations θi= A exp(-ti/τ) sin(ωti + φ) (i = 1, 2, 3, 4) by means ofcomputer. Therefore, not only can variations of period be measured directly, the variations of amplitude A, damping time τ and initial phase φ can also be calculated. Some modifications have been made on the system of light-electric accepter. Now, the precision detecting period with our torsional pendulum is one-hundred times higher than that of Dr. Saxl's as reported in his first article concerning pendulum in 1964, and ten times higher than the precision of his pendulum in 1970, which is the result of seventeen years of efforts in making improvements. The effective numbers obtained with our setup have reached six to seven figures, and the relative error of period is 2×10-6, or even 8×10-7 in the best case. Basically, the desired requirements have been satisfied. This will provide the possibility of detecting gravitational abnormal phenomena and making a distinction between the frequency fluctuation during a long period of time and that during a short period of time.