The utility model provides a tunnel experiment device with an adjustable cross section. The tunnel experiment device comprises a side wall framework unit, a road surface unit and a flexible layer, Wherein the side wall framework units are detachably and fixedly connected end to end in sequence to form a tunnel side wall framework, the pavement units are spliced in a staggered manner and are fixedto form a tunnel pavement, and the flexible layer is fixedly arranged on the inner side wall of the tunnel side wall framework; The cross section size, the cross section shape and the like of the tunnel can be adjusted according to actual needs, so that experimental tests can be carried out on environments in the tunnel with different cross section sizes and cross section shapes of the tunnel, andaccurate experimental parameter guidance is provided for actual tunnel manufacturing.