The difficulties in dyeing have significantly limited the application of alginate fibers in the field of textile industry. This study aimed to provide a dyeing method suitable for alginate fibers with reactive dyes. The results showed that alginate fibers could be successfully dyed with Reactive Red 195, Reactive Blue 19, and Reactive Yellow 145. Scanning Electron Microscope (SEM), Fourier Transform Infrared Spectrometer (FTIR), and fiber strength were investigated and found that dyed alginate fibers can keep their original structure and physical properties well. Colorfastness was also evaluated, which proved that dyed alginate fibers could maintain their color after soap washing, rubbing, and solarization to some extent. In order to further explore the applicability of the dyeing method, alginate fibers and lyocell were blended at 20/80 ratio to make yarns, knitted into weft plain fabrics, and carried out in the same way. The leveling property of dyed blended fabrics was good, for there was almost no color difference in naked eye observation. The washing and light fastness of blended fabrics were excellent, up to 4–5, while the rubbing fastness was slightly poor due to the fibrination of lyocell possibly. The weight loss rate of alginate fibers was less than 5%, which once again confirmed the dyeing method has less damage to alginate fibers. In addition, tensile strength of dyed blended fabrics was improved. This study can provide a new reference for alginate fiber dyeing.