Green, newly felled bamboo stems are prone to cracking during the drying process due to the growth stress and moisture gradient. To improve the drying quality and dimensional stability of bamboo stems, this study applied hygrothermal treatment under atmospheric pressure to newly felled bamboo stems. The temperature, relative humidity, and duration of the treatment were optimized using an orthogonal L₉ (3⁴) experimental design. The results show that the surface color of round bamboo became more uniform after hygrothermal treatment. Furthermore, hygrothermal treatment could reduce the cuticular wax and silicon layer detachment on the surface of round bamboo after drying. According to the range and variance analysis, the relative humidity had the greatest impact on dimensional stability, followed by treatment duration, whereas the temperature had a limited effect. The swelling rate of round bamboo under a hygrothermal treatment at a relative humidity of 95%, a temperature of 95 °C, and a duration of 3 h was decreased 53.72% and 62.76% compared with untreated round bamboo after moisture or water absorption for 7 d, respectively. However, no significant difference was observed in the color of the round bamboo under different hygrothermal treatment conditions. Overall, this study suggests that hygrothermal treatment could be a highly promising technology for improving the dimensional stability of newly felled bamboo stems.