A trailer with no extra height when folding, comprising fixed standpipes arranged at a front and rear end of the trailer, and front and rear folding components, side folding components, bottom folding components and handle components connected to the fixed standpipes; wherein a top-corner fixed piece is fixed on each fixed standpipe, each fixed standpipe is slidably connected with a first linkage sliding sleeve; the front and rear folding components comprise at least two hinged front and rear X components, the front and rear X components are formed by two cross pipes which are crosswise hinged together by an articulated shaft, the ends of the cross pipes are hinged with the top-corner fixed piece or the first linkage sliding sleeve; characterized in that the side folding components comprise two side folding shelves and a folding driving pipe, the side folding shelf comprises a side cross short pipe, a side cross long pipe and a horizontal connecting pipe, the side cross short pipe and the side cross long pipe are crosswise hinged by articulated shaft; one end of the folding driving pipes is hinged with two said horizontal connecting pipes, and the other end thereof is hinged with ends of two said side cross long pipes; the other end of the side cross long pipe is hinged with the top-corner fixed piece, one end of the side cross short pipe is hinged with the first linkage sliding sleeve and the other end thereof is hinged with one end of the horizontal connecting pipe; the bottom folding components comprise two bottom X components, the bottom X components are formed by one end of four bottom connecting pipes being simultaneously hinged with one same bottom connecting piece; the other end of the bottom connecting pipe is hinged with a second linkage sliding sleeve slidably connected with the folding driving pipe or a bottom-corner fixed piece fixedly connected to the fixed standpipes. wherein a top-corner fixed piece is fixed on each fixed standpipe, each fixed standpipe is slidably connected with a first linkage sliding sleeve; the front and rear folding components comprise at least two hinged front and rear X components, the front and rear X components are formed by two cross pipes which are crosswise hinged together by an articulated shaft, the ends of the cross pipes are hinged with the top-corner fixed piece or the first linkage sliding sleeve; characterized in that the side folding components comprise two side folding shelves and a folding driving pipe, the side folding shelf comprises a side cross short pipe, a side cross long pipe and a horizontal connecting pipe, the side cross short pipe and the side cross long pipe are crosswise hinged by articulated shaft; one end of the folding driving pipes is hinged with two said horizontal connecting pipes, and the other end thereof is hinged with ends of two said side cross long pipes; the other end of the side cross long pipe is hinged with the top-corner fixed piece, one end of the side cross short pipe is hinged with the first linkage sliding sleeve and the other end thereof is hinged with one end of the horizontal connecting pipe; the bottom folding components comprise two bottom X components, the bottom X components are formed by one end of four bottom connecting pipes being simultaneously hinged with one same bottom connecting piece; the other end of the bottom connecting pipe is hinged with a second linkage sliding sleeve slidably connected with the folding driving pipe or a bottom-corner fixed piece fixedly connected to the fixed standpipes.