Abstract: Through analyzing radiosonde observation data during pre-onset and onset period of monsoon, the authors find that the second tropopause (ST) (also called tropical tropopause) is the main tropopause in western Tibetan Plateau. This kind of characteristic can be seen in the two seasons. Double tropopause frequency is high in winter, while the first tropopause (FT) (also called polar tropopause) is rarely seen in summer, in which there is only the second tropopause. Before the monsoon onset, the height of FT is about 10752 m, with its temperature about 219 K, pressure 245.2 hPa; the height of ST is about 16826 m, with its temperature about 202 K, pressure 93 hPa. During the monsoon onset, the height of FT is 10695 m, with its temperature 229 K, pressure 257 hPa; the height of ST is 17360 m, with temperature 198 K, pressure 89.4 hPa. The difference of average temperature between the two periods demonstrate that the temperature of ST is high in winter and low in summer; the FT's temperature is high in summer, low in winter. At hourly scale, the second tropopause height is in opposite phase separately with its temperature, pressure, and wind speed, which means that higher tropopause height is accompanied with lower pressure, temperature, wind speed. The FT's inversion layer inhibits upward heat transporting and cloud height. The FT plays an important role in atmosphere heating. From uniform mixing of wind speed, the authors can conclude that boundary layer height can be as high as 3500 m in May and July. The westerly jet have obvious seasonal change. It is stronger in winter with hardly any easterlies. During the monsoon onset period, the westerly jet becomes weaker due to its moving away from Gaize to the north. While the easterlies becomes stronger due to moving nearer to Gaize, and is on top of the subtropical westerlies.