Hydrogels are increasingly used in medicine due to their potential to be delivered into the body in a minimally invasive manner and to be gelated at the site of introduction subsequently. The aim of this study was to develop a novel injectable and in situ-forming gel composite (GC) comprised of calcium alginate hydrogel and nano-hydroxyapatite/collagen (nHAC), assess its rheological, mechanical and in vitro degradable properties, and discuss the gelation mechanism. Injectable property test showed that the injectability of GC was tunable. Rheological results indicated that three phases of pre-gel, sol–gel phase transformation and post-gel could be found in the process of gelation. The compressive elastic modulus (E) and shear modulus (G) are in the range of 17.0–56.0 kPa and 24.7–55.0 kPa, respectively. During the in vitro degradation, the wet weight increased in the first week, then declined in the following 3 weeks, but the dry weight lost continuously during whole study. Meanwhile, the surface changed greatly after 2 weeks, but samples did not break down up to 28 days. These data indicate that GC exhibits controllable initial setting time and final setting time, tunable injectability, which provides a possible injectable material for bone repair and bone tissue engineering.