Polysaccharides were one of the important active ingredients of medical fungi, on account of its unique role caused the widespread concern of researchers at home and abroad. Currently, the structure and antioxidant activity of Coprinus comatus (C. comatus) fruit bodies polysaccharides have been reported. However, there is no article on the simultaneous study of intracellular and extracellular polysaccharides of liquid fermented C. comatus (IPs, EPs) so far. Hence, this paper innovatively investigated the monosaccharide compositions, antioxidative activities and inhibition of α-amylase activity in vitro of IPs and EPs. In this study, we adopted the PMP pre-column derivatization combined with HPLC method to determine the monosaccharide compositions of the IPs and EPs. Besides, the total antioxidant capacity, DPPH radical scavenging capacity, ·OH radical scavenging ability and inhibition effect on α-amylase in vitro of crude polysaccharides (ICPs, ECPs) and deproteinized polysaccharides (IDPs, EDPs) were severally compared. Only three kinds of monosaccharides were detected in IPs. Among the monosaccharides tested, the EPs contained five kinds, of which galactose and xylose accounted for a large proportion. ICPs showed better antioxidant activity and inhibition of α-amylase activity. This article innovatively studied the IPs and EPs, which could lay the foundation for the deep development of C. comatus.