The invention discloses an expansion force testing mechanism. The mechanism comprises a bracket and a reaction frame, a bedplate is arranged on the bracket, a reaction barrel with an opening in the top is fixedly arranged on the bedplate, a grouting opening is formed in the bottom end of the reaction barrel, a valve is arranged on the grouting opening, a piston which moves along the axial direction of the reaction barrel is arranged in the reaction barrel, a first connecting rod is arranged on the piston, a force value measuring element and a displacement measuring element are arranged on thefirst connecting rod, the top end of the first connecting rod extends to the reaction frame and is connected with the reaction frame, the lower portion of the reaction frame movably penetrates throughthe bedplate, and a loading device is arranged in the lower portion of the reaction frame. The expansion force of an expansion object under different loads can be measured, and the measurement precision is high. The mechanism is applied to the technical field of geotechnical engineering.