In the early stages of underground chamber formation, where the earthquake observation instruments are located, the chamber floor will exhibit the phenomenon of floor heave. Being a slow process, it will seriously affect the long-time change of ground tilt observation, and thus cause misjudgment in earthquake monitoring. In this study, through the numerical simulation of an underground stress chamber, we propose a stress field distribution law in the surrounding rocks of the chamber, which is obtained in the Z-direction deformation values of the grid nodes at the chamber floor. Then these deformation values are converted to tilt observation data. The results show that the change of the maximum principal stress field at the chamber floor will affect the horizontal pendulum of the seismograph. The tensile stress field varies at different degrees with different instrument positions. This irregular gradient tensile stress change in the vertical direction leads to the floor heave phenomenon, which results to the change of ground tilt in the chamber. The influence range of the ground tilt is between 17.46 and 8 177.02/10^-3″ arc-seconds. This study can help in meeting floor support requirements and the basis for a newly-built chamber, in order to better serve earthquake research in China.