Haematococcus pluvialis is an important source of natural astaxanthin, which plays a key role in the regulation of color, antioxidant capacity, and immunity in aquatic animals. However, it is unclear that the effects of long-term H. pluvialis astaxanthin feeding on the growth performance, coloration, and antioxidant capacity of commercial-sized rainbow trout Oncorhynchus mykiss. The commercial feed of O. mykiss was used as the basic feed, and 15, 30 mg/kg H. pluvialis astaxanthin were added to prepare isonitrogenous and isolipidic experimental diets, defined as HPA0, HPA15, and HPA30, respectively. Juvenile O. mykiss with an initial weight of 0.60 kg were fed with three experimental diets for 4 months, which was conducted to investigate the effects of long-term H. pluvialis astaxanthin feeding on the growth performance, coloration, antioxidant capacity, and biochemical composition. The results showed that the weight gain rate and specific growth rate of the HPA30 from 0 to 4 months were significantly higher than those of the other two diet groups. In the 4th month, the viscerosomatic index of O. mykiss significantly increased with increasing dietary astaxanthin levels. The redness (a*), DSM SalmoFan™ scores, and astaxanthin content of fillets significantly increased with increasing dietary astaxanthin levels during the whole feeding trial. The activities of total antioxidant capacity and glutathione peroxidase in the serum and liver, and serum immunoglobulin (IgM) content significantly increased with increasing dietary astaxanthin levels, while the levels of total antioxidant capacity, and malondialdehyde in the serum and liver showed a decreasing trend. The contents of total lipids in the fillet of O. mykiss increased significantly with increasing dietary astaxanthin levels, while the percentages of total saturated fatty acid (∑SFA), C18:3n3, and n-3/n-6 PUFA ratio showed a decreasing trend; the percentages of C18:2n6 and total n-6 polyunsaturated fatty acids (∑n-6PUFA) in the HPA15 and HPA30 were significantly higher than those of the HPA0. In conclusion, four months of feeding with 30 mg/kg H. pluvialis astaxanthin diet could improve the growth performance, fillet coloration, antioxidant capacity, and fillet total lipids content of commercial-sized O. mykiss. •The Haematococcus pluvialis is used to be dietary natural astaxanthin resource of commercial-sized Oncorhynchus mykiss.•Dietary H. pluvialis improved the growth, coloration, antioxidant capacity and fillet total lipids content of O. mykiss.•Fillet visual color of O. mykiss met the requirement of market after two months 30 mg/kg of H. pluvialis astaxanthin feeding.