Abstract: The research result of the indoor base line of Anqiu seismographic station in 16 years shows that the measurement precision of the indoor base line can reach 2×10^-7. A large part of the indoor base line variation is cursed by ground surface thermal deformation, with its value reaching 7×10^-5. The information of seismic activity is only about 5×10^-6, Thus decreasing and eliminating the ground surface thermal deformation is the key to improve the applied value of indoor base line.