Abstract: Taking into account the effect of moisture, we derive a three-dimensional pseudoenergy wave-activity relation for moist atmosphere from the primitive zonal momentum and total energy equations in Cartesian coordinates by using Energy-Casimir method. In the derivation, a Casimir function is introduced which is a single-value function of virtual potential temperature. Since the pseudoenergy wave-activity relation is constructed in the ageostrophic and nonhydrostatic dynamical framework, it may be applicable to diagnosing the stability of mesoscale disturbance systems in a steady-stratification atmosphere. The theoretical analysis shows that the wave-activity relation presented a nonconservative form in which the pseudoenergy wave-activity density is composed of disturbance kinetic energy, available potential energy and buoyant energy. The local change of pseudoenergy wave-activity density depends on the combination of shear of zonal basic flow, Coriolis force work and wave-activity source or sink as well as wave-activity flux divergence. The diagnosis shows that horizontal distribution and temporal trend of pseudoenergy wave-activity density are similar to that of the observation of 6-hour accumulated surface rainfall, which indicates that the pseudoenergy wave-activity density in the middle and low levels is capable of representing the dynamical and thermodynamic typical vertical feature of precipitable mesoscale systems so that the pseudoenergy wave-activity density is closely related to the observed surface rainfall. The calculation of the terms in wave-activity relation reveals that the wave-activity flux divergence shares a similar temporal trend with the locale change of pseudoenergy wave-activity density and the observed surface rainfall. Although the terms associated with the shear of zonal basic flow and the Coriolis force make contribution to the local change of pseudoenergy wave-activity density, the contribution coming from wave-activity flux divergence is much more noticeable.