Objective: To investigate the effect of changes in sodium ion concentration on the onset time of lidocaine in obstetric epidural block test doses. Methods: Eighty pregnant women scheduled for elective cesarean section, with no age or weight restrictions and ASA grade I or II, were selected. Lidocaine was diluted to two concentrations of 1% and 1.5% using normal saline (NS) and sterile water for injection (SW). The patients were randomly divided into four groups (n = 20): Group A (1% SW), Group B (1% NS), Group C (1.5% SW), and Group D (1.5% NS). A 3 mL test dose of epidural block was administered. The onset time, sensory and motor block levels, and adverse reactions were observed in each group. Results: The onset time was slowest in Group A (mean onset time 4.79 ± 0.65 minutes) and fastest in Group D (mean onset time 3.59 ± 0.61 minutes). Comparison between groups showed that the onset time was significantly shorter in Group B compared to Group A (P < 0.05), but there was no significant difference compared to Group C (P > 0.05). Group C had a faster onset time compared to Group A (P < 0.05), but a slower onset time compared to Group D, with a statistically significant difference. Group D had the fastest onset time, which was statistically significant compared to the other three groups. There were no significant differences in sensory and motor block levels or adverse reaction rates between the four groups (P > 0.05). Conclusion: Compared to sterile water for injection, diluting lidocaine with normal saline can shorten the onset time of the test dose. The 1.5% normal saline group had the shortest onset time, which is related to the increased sodium ion concentration in the solution, thereby reducing the onset time of lidocaine.