Abstract：There is P-wave in the rock simultaneously in S-wave measurement. The amplitude of P-wave is lower，but its velocity is higher than that of S-wave，so that it is difficult to decide the initial time of S-wave reaching. However，the velocity of S-wave in a rock block with a large plane can be measured with the probes for P-wave. Two probes for P-wave are parallelly coupled in a distance on the same plane of the rock block with grease. The transmitting probe produces a pulse which causes a P-wave in the normal direction of the plane and a S-wave in its shear direction. The vibration direction resulted from the S-wave is perpendicular to the plane，which is received by the probe for P-wave. For the end of probe is a circle，but not a point，the value is the upper limit of S-wave velocity obtained from the distance between the central points of two probes，and the lower limit can be obtained from the distance between the inner boundary of two probes.
