The invention discloses a single-system shell-and-tube evaporator of a heat pump unit. The evaporator comprises a shell, a copper tube, a front end cover, a rear end cover and turbulence plates; the shell is a cylindrical shell body, and is provided with a water inlet, a water outlet, an exhaust hole and a drainage hole; the inner wall surface of the copper tube is smooth, the outer wall surface of the copper tube is threaded, and the copper tube serving as a medium channel is divided into two systems; the outer circumference of the rear end cover is installed to block the rear end of the shell through bolts, and a gasket is arranged between the rear end cover and the shell; and each turbulence plate is a segmental circle with the radian exceeding the radian of a semicircle, and the segmental circles of every two adjacent turbulence plates are vertically located on the upper side and the lower side correspondingly. According to the evaporator, all the space of the whole evaporator is occupied by the shell, therefore, the water inlet and the water outlet of water and a medium are pre-arranged on the shell so that a large amount of space can be saved, and the design of a main engineis optimized; the water inlet and the water outlet are conveniently connected and installed through flanges, and maintenance and repair are easy; and the use frequency of the exhaust hole and the drainage hole is not high, the failure rate is very low, and therefore, the leakage can be reduced by adopting a welding method.