The invention belongs to the field of cable working wells, and specifically discloses an assembled cable working well and an on-site construction method. A plain concrete cushion, a reinforced concrete bottom plate, U-shaped reinforced concrete side plates and a reinforced concrete cover plate are included, the reinforced concrete bottom plate is placed on the plain concrete cushion after foundation excavation, rigid cushion blocks are placed on the reinforced concrete bottom plate, then the reinforced concrete side plates are hoisted to be placed on the rigid cushion blocks, formwork buildingand pouring on a constructional column are carried out after steel bars constructed on the ends of the U-shaped reinforced concrete side plates and longitudinal reinforcements are tied, and the reinforced concrete cover plate is laid aside on a transverse beam of the U-shaped reinforced concrete side plates when the concrete strength reaches 75% of the design value. The assembled cable working well has the advantages of being reasonable in structure, good in anti-seepage performance, simple to assembly, short in construction period, low in cost, small in environmental disturbance and the like.