Using the data of natural earthquakes, we have studied the characteristics of P-wave travel time in Qinghai-Xizang plateau and its vicinity. From 2° to 26°, travel time of p-wave could be approximately described by three sections of straight line, there are remarkable changes for apparent velocities near 10 and 19° Inferred upper mantle structure shows:
(1)The P-wave velocity in the uppermost part of the mantle beneath the plateau is 8.11±0.04 km/sec;
(2)In the Tengchong(腾冲)district, there exists a low-velocity layer with a p-wave velocity of 7.59±0.09km/sec. The depth of the low velocity layer is between 40km and 170km;
(3)The depth of "20° discontinuity" is 450-500km, and the P wave velocity beneath it is about 9.9km/sec;
(4)P-wave velocities increase, with depth from Moho towards "20± discontinuity".
Besides, we have discussed the tectonophysical implications of the results in this paper.