The invention relates to a locating device used for drilling and milling of a motor aluminum part long shell. The motor aluminum part long shell comprises a square-table-shaped top (10), a bottom (11) and a connection part (13). One side of the bottom (11) is in the shape of a V. The connection part (13) connects the top (10) with the bottom (11) and is provided with flanks. The locating device comprises pressing plates (1), a locating plate, a connection plate (5) and a V-shaped locating seat (9). The locating plate is arranged at one end of the connection plate (5). The V-shaped locating seat (9) is installed on the connection plate (5). The motor aluminum part long shell is place on the connection plate (5). The bottom (11) of the motor aluminum part long shell is fixed in the locating plate. The square-table-shaped top (10) of the motor aluminum part long shell leans on V-shaped locating seat (9). The connection part (13) of the motor aluminum part long shell is fixed to the connection plate. Compared with the prior art, the locating device has the advantages that the machining stability is good, the machining quality is high, and the adjustability of a to-be-machined part is high.