To study the effect of multi-point initiation circuit charge on detonation wave output synchronicity and initiation capability, a rigid three-point synchronous initiation circuit was designed using ultrafine Octogen (HMX) as main explosive, nitrocellulose (NC) as binder, 95/5-HMX/NC as groove booster charge. The synchronicity error of initiation circuit at different charge density was theoretically analyzed and experimentally measured. The groove press loading charge technique was compared and tested. The booster explosive was characterized, the detonation performances related to initiation circuit were tested and the initiation circuit charge structure was optimized. Results show that increasing circuit charge density can increase the initiation reliability and reduce the synchronicity error. The synchronicity error of initiation circuit decreases from 300 ns to 150 ns when the charge density increases from 1.17 g·cm^-3 to 1.47 g·cm^-3. Taking pressed JH-2 cylinder charge as output charge and ultrafine HMX/NC as groove booster, use of groove press loading charge technique can make the detonation wave output synchronicity of synchronous initiation circuit as about 100 ns.