The invention discloses a three-dimensional gradient boundary transition device applied to wind field characteristic wind tunnel experiments of a terrain model. the boundary transition device is mainly formed by serially connecting and splicing a plurality of sections; each section comprises two side plates and a surface layer; each side plate comprises a vertical edge, a bottom edge and a curve edge; the surface layer is fixedly paved on the curve edges of two corresponding side plates; the vertical edges of the side plates are higher than positions corresponding to the boundaries of the terrain model; the lengths of the bottom edges of the side plates are 5 to 8 % of the width of a wind tunnel; the gradient between the vertical edge of each side plate and the bottom edge of the side plate is 20 to 40 degrees; the line font of the curve edge of each side plate is defined according to the equation that y-r<2>/y+r<2>x<2>/y<3>-r<2>x<4>/y<5>+r<2>x<6>/y<7>-r<2>x<8>/y<9>-m=0, wherein x represents the radial position of each curve edge, y represents the vertical position of the curve edge, the parameters r and m are defined according to the equation set shown in the specification, h0 represents the height of each vertical edge, k0 represents the ratio between the height of the vertical edge and the length of each bottom edge. Through the adoption of the boundary transition device, under the premise that a wind field characteristic meets requirements after airflow transition, the requirement of wind tunnel space is low, and the applicability is high.