Haematococcus pluvialis is the predominant natural source of astaxanthin, yet its large-scale production is constrained by bottlenecks such as low efficiency and high energy consumption in suspension culture, as well as high cost, poor mechanical properties and easy dehydration of carriers in existing immobilization technologies. To establish a high-performance and low-cost immobilized culture system, this study designed and optimized a composite gel carrier based on low-concentration sodium alginate. By systematically optimizing the concentration of calcium chloride cross-linking medium, the mass ratio of sodium alginate to calcium carbonate, and introducing glycerol as a water-retaining agent coupled with polyethylene film covering technology, an immobilized gel with favorable mechanical strength, light transmittance and water retention capacity was successfully fabricated. The results demonstrated that within the 7-day culture period, the optimal system achieved a water loss rate as low as 49.90%, representing a 23.10% reduction compared with the non-film-covered group; the astaxanthin yield reached 10.787 mg, a 32.68% increase relative to the group without water-retaining agent. This optimized system effectively addresses the core issues of fragility, poor light transmittance and rapid water loss of immobilized carriers, providing a novel technical solution for the efficient and low-cost large-scale production of astaxanthin from Haematococcus pluvialis.