A heat exchanger, comprising: a plurality of adjacent sheets, wherein the adjacent sheets extend parallel to one another along the same direction; wherein the adjacent sheets are connected to one other at least at a part of their peripheral edges or middle portions, and flow channels for fluid flow are formed between the adjacent sheets; wherein an upper surface of each sheet is provided with a plurality of raised peak lines that are arranged in parallel and spaced rows and raised upward, and a lower surface of each sheet is provided with a plurality of raised contour lines that are arranged in parallel and spaced rows and raised downward, wherein the raised peak lines and the raised contour lines that are arranged in rows are both connected by continuous peaks and valleys; wherein between adjacent three sheets, the peaks of the raised contour lines are located between two valleys of the raised peak lines, and the peaks of the raised contour lines and the two valleys of the raised peak lines are staggered, to form a first flow channel and a second flow channel; wherein fluid flows through the first flow channel along a first direction and a second direction, and fluid flows through the second flow channel along the first direction and a third direction, to form convection by the fluid flowing through the first flow channel and the second flow channel; and the first direction is arranged at an angle with respect to the second direction and the third direction, and the second direction and the third direction are parallel and opposite to each other; characterized in that a trajectory formed by the fluid flowing along the first direction is a curve, the curve having alternating peaks and valleys; a trajectory formed by the fluid flowing along the second direction or the third direction is a straight line, the second direction and the third direction being parallel and opposite to each other; and a horizontal angle between the first direction and the second direction, and between the first direction and the third direction is α, and α is ranged from 30° to 90°. wherein the adjacent sheets extend parallel to one another along the same direction; wherein the adjacent sheets are connected to one other at least at a part of their peripheral edges or middle portions, and flow channels for fluid flow are formed between the adjacent sheets; wherein an upper surface of each sheet is provided with a plurality of raised peak lines that are arranged in parallel and spaced rows and raised upward, and a lower surface of each sheet is provided with a plurality of raised contour lines that are arranged in parallel and spaced rows and raised downward, wherein the raised peak lines and the raised contour lines that are arranged in rows are both connected by continuous peaks and valleys; wherein between adjacent three sheets, the peaks of the raised contour lines are located between two valleys of the raised peak lines, and the peaks of the raised contour lines and the two valleys of the raised peak lines are staggered, to form a first flow channel and a second flow channel; wherein fluid flows through the first flow channel along a first direction and a second direction, and fluid flows through the second flow channel along the first direction and a third direction, to form convection by the fluid flowing through the first flow channel and the second flow channel; and the first direction is arranged at an angle with respect to the second direction and the third direction, and the second direction and the third direction are parallel and opposite to each other; characterized in that a trajectory formed by the fluid flowing along the first direction is a curve, the curve having alternating peaks and valleys; a trajectory formed by the fluid flowing along the second direction or the third direction is a straight line, the second direction and the third direction being parallel and opposite to each other; and a horizontal angle between the first direction and the second direction, and between the first direction and the third direction is α, and α is ranged from 30° to 90°.