The invention discloses an assembly tooling for a diesel engine valve collet, which comprises a forked pressure head, wherein an opening of the forked pressure head is smaller than a horizontal size of a valve spring upper base and used for pressing down the valve spring upper base, and the opening of the forked pressure head is at least larger than a stem outer diameter of a valve stem for installing the valve collet. In use, a pressure towards the valve spring upper base is applied to the forked pressure head, the spring is compressed and three ring slots are exposed, three annular bulges on two flaps of the valve collet are embedded in three rings and the valve collet is formed as one piece, the forked pressure head releases the valve spring upper base, and the valve spring upper base is externally sleeved on the valve collet outer circle under the spring action. With the assembly tooling, positioning and assembling can be rapidly carried out; the coaxiality of the valve collet, the valve spring upper base and the valve stem can be ensured; the direction of the spring force can be made to meet the stem movement requirement; the assembled valve system can be ensured to accord with the operating condition of the diesel engine; the working efficiency in assembly can be increased; and the assembly tooling is especially suitable for the assembling requirement of batch production of multi-valve diesel engines.