The COMET Phase-I experiment aims to search for the muon-to-electron conversion process with a target single event sensitivity of $3 \times 10^{-15}$. The main detector, consisting of Cylindrical Drift Chamber (CDC) and Cylindrical Trigger Hodoscope (CTH), requires an online system capable of high-rate, low-latency operation, while also being sufficiently flexible and robust. This contribution reports on the development status of the online system, including the scalable asynchronous data acquisition framework, the integrated slow control system, and the multi-stage hardware trigger system. We also present the results of full hardware integration tests for the CDC trigger system, focusing on communication stability over long-distance connections with DisplayPort cables and end-to-end trigger latency measurements. The system operated for about 18 hours without communication errors, and the total trigger latency is confirmed to satisfy the 8.5 $\mu$s requirement.