This study was conducted to investigate the effects of rumen-protected tryptophan (RPT) on Cashmere goats during the cashmere slow growth period. Optimal RPT supplementation level was determined. The Thirty-six Liaoning Cashmere goat wethers (41.63 ± 0.37 kg) aged 1.5-year-old were stratified by weight and randomly assigned to the following treatments: without RPT (control), low RPT (LRPT), medium RPT (MRPT) and high RPT (HRPT) at RPT levels of 0, 2.0, 4.0 and 6.0 g per goat per day, respectively. LRPT and MRPT supplementation increased average daily gain (P < 0.05) and N retention (P < 0.05).The length and growth rate of cashmere fibre increased both linearly (P < 0.05) and quadratically (P < 0.05) with increasing RPT supplementation. Plasma Trp concentration increased linearly (P < 0.05) with increasing RPT supplementation. Conversely, RPT supplementation decreased plasma isoleucine, valine and total essential amino acid concentrations (P < 0.05). In conclusion, RPT supplementation potentially enhanced growth performance, cashmere fibre growth and N utilization in Liaoning Cashmere goat wethers. In experimental conditions of the current trial, the optimum RPT supplementation level was 2.0 g per goat per day during the cashmere slow growth period.