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The ORBIT code has been widely used for beam dynamics simulations includinginjection and acceleration in high-intensity hadron synchrotrons. When theORBIT's 1D longitudinal tracking was employed for the acceleration process inCSNS/RCS, the longitudinal emittance in eV-s was found decreasing substantiallyduring acceleration, though the adiabatic condition is still met during thisprocess. This is against the Liouville theorem that predicts the preservationof the emittance during acceleration. The recent machine study in theaccelerator and the simulations with a self-made code demonstrate that thelongitudinal emittance is almost invariant, which further indicates that theORBIT longitudinal tracking might be incorrect. A detailed check-over in theORBIT code source finds that the longitudinal finite difference equation usedin the code is erroneous when applied to an acceleration process. The new codeformat PyORBIT has the same problem. After the small secondary factor isincluded in the code, ORBIT can produce results keeping the longitudinalemittance invariant. This paper presents some details about the study.

Problem and solution with the longitudinal tracking of the ORBIT code