Objective  To investigate the differences in positive relative accommodation （PRA） and negative relative accommodation （NRA） in juvenile emmetropes and myopes， and to analyze the relationship between the differences in PRA and NRA and the onset of myopia. Methods  In this cross-sectional study， the subjects were divided into two groups， a juvenile group and young adult group （entrant group）. The juvenile group included 43 emmetropes （+0.5~-0.25 D， 7-16 years old， average age 10.7±3.0 years） and 110 myopes （-0.5~-3.00 D， anisometropia less than -1.00 D， 7-16 years old， average age 11.5±2.6 years）. There were 266 participants （average age 18.76±0.80 years） in the youth group， including 29 emmetropes and 237 myopes. PRA and NRA were measured by a phoroptor based on computer optometry. A one-way ANOVA was used to analyze the comparisons between these two groups. Results  In the juvenile group， PRA was -5.52±1.21 D for emmetropes compared to -2.27±1.19 D for myopes， which was a statistically significant difference （F=227.26， P＜0.01）. The NRA for emmetropes was +2.41±0.65 D and was +2.21±0.73 D for myopes. The difference was not statistically significant. The PRA/NRA ratios for emmetropes and myopes were 2.35±0.45 and 1.10±0.60， and the difference was statistically significantly （F=156.40， P＜0.01）. In the young adult group， PRA was -4.87±1.47 D for emmetropes in comparison to -2.63±1.41 D for myopes， which was a statistically significant difference （F=66.62， P＜0.01）. The NRA of emmetropes was +2.09±0.46 D and was +1.98±0.48 D for myopes. The difference was not statistically significant （F=1.55， P=0.214）. The PRA/NRA ratios for emmetropes and myopes were 2.35±0.51 and 1.34±0.93. The difference was statistically significant （F=47.28， P＜0.01）. Conclusion  The PRA and PRA/NRA ratio for juvenile and young adult myopes is significantly less than for emmetropes. The decrease in PRA， i.e.， accommodation reserve， is a significantly different index between myopes and emmetropes in juveniles and young adults.