The invention discloses a bent cap assembling assisting device and a construction method thereof. The device comprises an upper embedded part and a lower limiting part, the embedded part is preset atthe bottom of a prefabricated bent cap, and the limiting part is fixed to a prefabricated vertical column; the embedded part comprises an embedded sleeve and a telescopic rod arranged in the embeddedsleeve; the limiting part comprises a broad-mouth sleeve, the top of the broad-mouth sleeve is provided with an inverted-cone-shaped broad-mouth opening, and a jacking device is arranged at the bottomof the broad-mouth sleeve; the telescopic rod is inserted into the broad-mouth sleeve from the broad-mouth opening and abuts against the top of the jacking device. The device has the advantages thatinstallation is convenient, the operation is simple and easy, the reliability of the device is high, the embedded part is arranged in a preset factory to reduce the site construction amount, and the working efficiency is improved; the problem of difficulty in alignment in the hoisting process of the prefabricated bent cap is solved, the hoisted prefabricated bent cap can be adjusted, and the assembling precision of the bent cap is improved.