Blue large-amplitude pulsators (BLAPs) represent a new and rare class of hot pulsating stars with unusually large amplitudes and short periods. Up to now, only 21 confirmed BLAPs have been identified from more than one billion monitored stars, including a group with pulsation period longer than 20 min (classical BLAPs; ) and the other group with pulsation period below 10 min (high-gravity BLAPs; ). The evolutionary path that could give rise to such kinds of stellar configurations is unclear. Here we report on a comprehensive study of the peculiar BLAP discovered by the Tsinghua University – Ma Huateng Telescopes for Survey (TMTS) , TMTS J035143.63+584504.2 (TMTSBLAP-1); it is the first confirmed BLAP located within the “period gap” between high-gravity and classical BLAPs. This new BLAP has an 18.9 min pulsation period, and is similar to the classical BLAPs with an extended helium-enriched envelope. In particular, the long-term monitoring data of TMTS-BLAP-1 reveal that this pulsating star has an unusually large rate of period change, ˙P /P = 2.3 × 10−6 yr−1. Such a significant and positive value challenges the scenarios of both the helium-core pre-white-dwarf and the core-helium burning [1, 2, 4–6]), but is consistent with that derived from shell-helium burning. The particular pulsation period and unusual rate of period change indicate that TMTS-BLAP-1 is at a short-lived (∼ 106 yr) phase of shell-helium ignition before the stable shell-helium burning; in other words, TMTSBLAP-1 is going through a “Hertzsprung gap” of hot subdwarfs. This demonstrates that hot subdwarf stars have a similar evolutionary fate as main-sequence stars; some subdwarfs can leave their “main-sequence” stage and appear as diverse pulsating variables in distinct regions of the Hertzsprung-Russell diagram, thereby opening a new window for explorations of rare pulsating stars that evolved from stripped-envelope stars.