The invention relates to a nozzle for a selective catalytic conversion system of a diesel engine. According to the technical scheme, a spray orifice is arranged on a spray head, a heat insulation body is sleeved outside the middle section of a perfusion tube, a heat insulation tube is sleeved outside the heat insulation body, an installation base is sleeved outside the heat insulation tube, the outer wall surface of the installation base is provided with an external thread and a fastening nut, the fastening nut is provided with an internal thread, the installation base is screwed inside the fastening nut, a positioning plate is arranged on the top end part of the heat insulation body, the positioning plate is sleeved outside the perfusion tube and is provided with a positioning bolt, the positioning bolt is arranged along the radial direction of the positioning plate and the outer end part of the positioning bolt protrudes from the outer wall of the positioning plate, a limit plate integrally connected with the fastening nut is arranged on the top end part of the fastening nut, and the limit plate is overlapped on the top end surface of the positioning plate. The invention has a favorable atomization effect, the design of the heat insulation can protect urea from crystallization to block pipelines, and the invention has the characteristics of simple installation, convenient repair, small size and suitable installation and use on exhaust pipes.