Abstract: To investigate the influences of size effect and in-situ stress of columnar jointed basalt relaxation zone, 8 typical tunnels with different sizes are selected, and an empirical formula for relaxation depth is established based on the data statistics and curve fitting of 161 groups of test results by using the single-hole acoustic wave tests. The test results show that the excavation section size and high in-situ stress are the two main influencing factors of the relaxation depth. The maximum relaxation depth appears in the middle-upper part of side wall of cavern, the size effect is obvious and linearly correlates with the height of cavern, and high in-situ stress will increase the size effect. Under the low in-situ stress, the relaxation depth is generally 1/6 of the height of cavern. Under the high in-situ stress, the relaxation depth is generally 1/3 of the height of cavern, and in the middle of 24 m-high cavern wall, the extreme relaxation depth will reach 10.2 m.