The NEOS searches for sterile neutrinos by detecting reactor antineutrinos at a very short baseline in Korea. The NEOS detectors (1-ton Gd-LS) were deployed at the tendon gallery of the Hanbit reactor unit 5 (2.8 GW thermal power), 24 m away from the reactor core. In NEOS-I, the prompt energy spectrum from inverse-beta-decay was measured using 180 live-days of reactor-on data, which clearly showed the "5 MeV excess". To understand the "reactor antineutrino anomaly" and the origin of the "5 MeV excess", NEOS-II has recorded 388 (112) live-days of reactor-on(-off) data from September 2018 to October 2020, covering a whole burnup cycle of the reactor. In this work, we report studies on the response of the NEOS-II detector, results of event reconstruction, and related corrections, e.g., for the change of the detector characteristics with time. A simulation was developed and compared with the data energy spectra of beta and gamma events, validating how simulation fits data well.