Ca3Co2-xMnxO6 (x≈1, CCMO) is a typical magnetic multiferroic material. The spins of Co2+ and Mn4+ have strong magnetic anisotropy, which are similar to Ising spins. Moreover, at low temperatures, CCMO forms a particular up-up-town-town (↑↑↓↓) spin order, the so-called E-type antiferromagnetic state. In this paper, a diatomic Ising chain model for CCMO system, with the nearest-neighbour exchange frustration, has been studied by Monte Carlo simulation. The zero-temperature phase diagram is calculated by the variational method, which contains the E-type antiferromagnetic phase. Monte Carlo simulations confirm the existence of E-type antiferromagnet. Based on the results of specific heat and spin structure factor, two phase transition temperatures have been revealed, which may correspond to two phase transitions in real CCMO. By continuously adjusting the intensity of nearest-neighbour exchange interaction in Monte Carlo simulations, the phase transition boundary between the G-type and E-type antiferromagnetic states shows critical phenomena. Near the phase boundary, the spin modulation period becomes incommensurate. The instability of E-type antiferromagnetic order in real CCMO with x=1, may just due to the low dimensional features of the 1D Ising spin chain and the exchange frustration.