Using the Institut de Radioastronomie Millmetrique 30m telescope, we presented observations of N_2_H^+^J=1-0, CCS J_N_=8_7_-7_6_ and 7_7_-6_6_ lines toward a large sample of ultracompact H II regions (UC H IIs). Among our 88 UC H IIs, 87 and 33 sources were detected in the N_2_H^+^J=1-0 and CCS J_N_=8_7_-7_6_ lines, respectively. For the CCS 7_7_-6_6_ transition, we detected emission in 10 out of 82 targeted sources, all of which also exhibited emission in the CCS J_N_=8_7_-7_6_ line. Physical parameters are derived for our detections, including the optical depth and excitation temperature of N_2_H^+^, the rotational temperature of CCS and the column density. Combining our results and previous observation results in different stages of high-mass star-forming regions (HMSFRs), we found that the column density ratio N(N_2_H^+^)/N(CCS) increases from high-mass starless cores through high-mass protostellar cores to UC H IIs. This implies that N(N_2_H^+^)/N(CCS) can trace the evolution process of HMSFRs. It was supported by our gas-grain chemical model, which shows that N(N_2_H^+^)/N(CCS) increases with the evolution age of HMSFRs. The temperature, density and chemical age were also constrained from our best-fit model at each stage. Thus, we propose N(N_2_H^+^)/N(CCS) as a reliable chemical clock of HMSFRs.

Cone search capability for table J/AJ/170/74/table1 (Detailed information of our Ultracompact (UC) HII sample)

Cone search capability for table J/AJ/170/74/table9 (Basic information of High-Mass Starless Cores (HMSC) and High-Mass Protostellar Cores (HMPO) from Chen+2025 (in preparation))