The invention belongs to mechanical parts and particularly relates to a spiral gear with the circular tooth profile and for a hydraulic device. The spiral gear comprises a driving circular spiral gear (1) and a driven circular spiral gear (2). The tooth profile (1-1 and 1-2) of the driving circular spiral gear and the driven circular spiral gear is composed of a tooth top protruding circular segment (BC), a tooth root recessed circular segment (EG) and a transition curve line segment (CE) between the tooth top protruding circular segment (BC) and the tooth root recessed circular segment (EG). The transition curve line segment (CE) is a combination line segment composed of a cosine curve line segment (CD) and a cosine curve conjugate curve line segment (DE) in tangency. The radius (AB) of the tooth top protruding circular segment is equal to the radius (FG) of the tooth root recessed circular segment. The tooth top protruding circular segment (BC) and the tooth root recessed circular segment (EG) are both in tangency with the transition curve line segment (CE). The meshing points move along the tooth profile (1-1 and 1-2) of the driving circular spiral gear and the driven circular spiral gear. The overlapping ratio of the driving circular spiral gear (1) and the driven circular spiral gear (2) is close to or equal to 1. The spiral gear with the circular tooth profile and for the hydraulic device prolongs constant volume output and improves pump efficiency.