A femtosecond laser system with a pulse repetition rate up to 1 kHz is being developed at the Institute of Laser Physics of the SB RAS. The key component of the system is the channel of laser amplification based on media doped with Yb3+ ions. The designed average output power of the channel is 300 W. Thermal effects in high-power laser amplifiers are decisive for the parameters of the radiation at the output of system. In this paper the results of experimental measurements of thermal lens focal distance dependence in Yb:YAG active element on diode pump power using geometric method are given. Calculation of the dependence thermal lens focal distance on the power of pump power and a comparison with experimental data are given.