To investigate the suppression effect of attached cavities on the overpressure load and its oscillation of gas explosion in confined tubes, the numerical models of gas explosion in pipelines was established based on the CFD software FLACS. The reliability of the model in predicting the overpressure load was validated using experimental data. Numerical simulations were conducted to analyze the effects of gas cloud length, cavity size, and cavity location on the overpressure and its oscillation characteristics. The mechanism of the attached cavity on gas explosion in pipelines was revealed. The proposed numerical model effectively predicts the overpressure load of gas explosion in the tube, and the numerical results are in good agreement with experimental data. For the confined tube with a cavity (square cross-section), the best suppression effect is achieved when the cavity length is equal to the cross-sectional dimension and the cavity is located at the middle of the pipeline. For the condition, it can reduce the overpressure oscillation amplitude by about 50% and the final explosion overpressure by about 17%. However, when the cavity is close to the ignition end or the cavity length is relatively large, the cavity has a significant promoting effect on the explosion in the tube. The attached cavity suppresses the explosion overpressure and its oscillation through wave damping, while the disturbance of the unburned gas would promote the gas explosion. The size and location of the cavity could determine whether the overpressure and the oscillation are promoted or suppressed.