In degenerative disc disease, an injectable hydrogel can fill a degenerate area completely, reduce the risk of implant migration and subsequent loss of height of the intervertebral disc, and minimise surgical defects. Here, we propose a method of preparing an injectable silk fibroin/polyurethane (SF/PU) composite hydrogel by chemical cross-linking under physiological conditions. Mechanical testing was used to determine the mechanical strength of the hydrogel. The impact of hydrogel height on the biomechanical properties was discussed to estimate the working capacity of the hydrogel for further clinical application. Rheological properties were also examined to assess the practical ability of the hydrogel for clinical application. Hydrogel injection and cell assessment is also of interest for clinical application. An SF/PU composite hydrogel can be injected through a small incision. A cell proliferation assay using bone marrow stromal cells showed positive cell viability and increased proliferation over a seven-day period in culture. Importantly, the hydrogel can be monitored in real-time using X-ray fluoroscopy during and after surgery according to the results of X-ray fluoroscopy examination, and shows good visibility based on X-ray assays. In particular, the hydrogel offers the clinically important advantage of visibility in CT and T2-weighted magnetic resonance imaging. Based on the results of the current study, the SF/AU composite hydrogel may offer several advantages for future application in nucleus pulposus replacement.