The invention relates to a posture measurement device and a measurement method of rectangular shield construction; the measurement device comprises at least two target prisms which are installed on a rectangular shield tunneling machine, two dual-shaft sensors which are symmetrically installed at centers on bilateral sides of the shield tail of the rectangular shield tunneling machine, a total station which is installed on a pipe section at the entrance of the tunnel; an industrial personal computer which is in communication connections with the dual-shaft sensors and the total station; the industrial personal computer receives and uses the azimuth angle, slope angle and roll angle of the rectangular shield tunneling machine which are measured by the dual-shaft sensors and coordinate information of the target prisms which is acquired by the total station, to calculate and obtain the existing left route axis and right route axis of the rectangular shield tunneling machine, thereby obtaining the posture information of the rectangular shield tunneling machine by respectively comparing with a stored left preset axis and a right preset axis. The invention provides a posture measurement method of rectangular shield construction; the method ensures the accuracy of the measurement and also has the advantages of simple in operation, and convenient for implementation.