Background: To evaluate whether a shorter time of lying supine without a pillow and fasting for solids and liquids (LSFSL) after a lumbar puncture (LP) is associated with a higher risk of post-lumbar puncture headache (PLPH) and post-lumbar puncture lower back pain (PLPBP) in a randomized, assessor-blinded, controlled trial. Methods: Paediatric patients who underwent their first LP after hospital admission were randomly allocated to either the group with half an hour of LSFSL (0.5 h LSFSL) or 4 h of LSFSL (4 h LSFSL) immediately after LP. The primary outcome is PLPH after LP. The incidence of PLPH, PLPBP, and vomiting; vital signs (respiratory rate, heart rate, blood pressure); and other post-procedure conditions after LP were measured as the outcomes. The Non-inferiority test and Wilcoxon rank-sum test were used to analyse the outcome data. Results: In total, 400 patients (201 in the 0.5-h LSFSL group and 199 in the 4-h LSFSL group) were included in this trial. Twelve (5.97%) of 201 patients experienced PLPH in the 0.5 h LSFSL group versus 13 (6.53%) of 199 patients in the 4 h LSFSL group (difference 0.56, 95% CI -4.18 to 5.31; p = 0.0108 for the non-inferiority test). Fourteen (6.97%) of 201 patients experienced PLPBP in the 0.5 h LSFSL group versus 17 (8.54%) of 199 patients in the 4 h LSFSL group (difference 1.57, 95% CI -3.66 to 6.82; p = 0.007 for the non-inferiority test). The changes in heart rate (HR), respiratory rate (RP) and systolic blood pressure (SBP) before and after the LP were not different between the 0.5-h LSFSL group and the 4-h LSFSL group. No other adverse events were reported. Conclusions: Compared with 4 h of LSFSL after LP, 0.5 h of LSFSL was not associated with a higher risk of PLPH, PLPBP or other adverse events. In conclusion, 0.5 h of LSFSL is sufficient for children undergoing LP.