Seeking two-dimensional materials with low lattice thermal conductivity (klat)is of great significance to enhance high thermoelectric performance. Based on ab-initio calculations and phonon Boltzmann transport theory, the structure of two-dimensional CdO is optimized and the dynamic stability of monolayer CdO is verified by calculating phonon spectra. The phonon transport properties of monolayer CdO were studied in detail. The results show that the lattice thermal conductivity of monolayer CdO is around 5.7 W/(m·K) at room temperature, which is much lower than that of monolayer graphene, buckled monolayer phosphorene, monolayer black phosphorene, and MoS2. The percentage contribution of Z-direction acoustic (ZA), transverse acoustic(TA), longitudinal acoustic(LA), Z-direction optical (ZO), transverse optical(TO) and longitudinal optical (LO) branches to klat is 73.7%, 13.9%, 3.7%, 2.8%, 4.7% and 1.2%, respectively. The strong scattering among optical-acoustic phonons is found to be responsible for the low thermal conductivity of monolayer CdO. The calculated results could guide the design of CdO-based low-dimensional thermoelectric devices.