In order to identify a suitable substitute for traditional gelatin soft capsule shells, a series of composite soft capsule shells with different glycerol concentrations were prepared based on a casting method, using hydroxypropyl starch (HPS) and agar (AG) as raw materials and glycerol (G) as a plasticizer. The effects of glycerol concentration on the properties of the composite capsule shells were investigated. Fourier transform infrared spectroscopic analysis revealed hydrogen bond interactions between HPS and AG in the composite capsule shell, whereas the findings of thermogravimetric analysis indicated that the addition of glycerol reduces the compatibility between HPS and AG, thereby leading to an increase in the thermal stability of the composite capsule shells with an increase in glycerol concentration (0~20%, m/m). Absorbance of the composite material at 600 nm initially increased and subsequently declined with increasing glycerol concentration, with maximum absorbance (0.60) being recorded for composite capsule shells with a 10% (m/m) glycerol concentration (10% G+HPS+AG), thus indicating good opacity. Similarly, the water contact angle (WCA) of the composite capsule shells initially increased with increasing glycerol concentration prior to a subsequent decline, with the highest value (86.17°) being obtained for the 15% G+HPS+AG composite capsule shells, indicating good hydrophobicity. The 15% G+HPS+AG composite capsule shells also had good Young’s modulus (17.47 MPa), tensile strength (3.17 MPa), and elongation at break (94.17%), thereby meeting the requirements for stable production. Compared with the traditional gelatin soft capsule shells, the 15% G+HPS+AG composite shells were softer under dry storage conditions and had good storage stability. Moreover, given that the raw materials are of non-animal origin, this would ensure better safety. Our findings indicate that the HPS-AG composite system has promising application prospects in the field of soft capsule shell production, and will provide a theoretical foundation for the industrial manufacture of starch-based soft capsule shells.