The effect of dietary supplementation of Flammulina velutipes concentrated extract (CE) on catfish fillet quality under cool preservation was evaluated. The catfish were fed by feed supplementing with 0% to 15% CE during the growth phase. After 30 days, the fish were filleted and stored at 4 ± 0.2°C for 7 days. The results showed that with 10% CE, the fish produced the best growth performance from 605 to 1106 g, corresponding to the lowest disease rate of 1.5%. Ergothioneine in muscle reached the highest value at 20.63 ± 1.35 mg/Kg. During the preservation, ergothioneine, 2,2-Diphenyl-2-picrylhydrazyl (DPPH) and lipid decreased slightly while free fatty acid (FFA) increased gradually until day 7. Peroxide and thiobarbituric acid reactive substances increased slowly after 7 days to 0.90 and 0.55 mg/Kg MDA, respectively. The b* value and total color variation (∆E) showed the slowest increase of −0.21 ± 0.67 and 46.85 ± 0.97, respectively, while the a* value decreased gradually during storage. pH decreased slightly on day 1, then increased to 6.4 ± 0.2 on day 7. The fillet was firmer on day 1, then decreased gradually in all treatments. Therefore, an increasing ergothioneine in catfish muscle leads to higher growth performance with a low disease rate and helps to prevent lipid oxidation, improving color and structural stability.