In this paper, a new class of four-membered catalyst system, PdCl2/MA(n)/DPPP1/CF3COOH, was firstly studied for copolymerization of CO with ethylene to prepare linear alternating polyketone, in which MA(n) component was mainly studied. The experimental results showed that this kind of new catalyst system exhibited highly efficient activity; especially when Cd(OAc)(2). 2H(2)O, Zn(OAc)(2). 4H(2)O, Fe(ClO4)(3). 9H(2)O, Pb(OAc)(2). 3H(2)O and Cu(OAc)(2).H2O were used as MA(n) component, the corresponding catalyst system had very highly efficient activity of 8100, 7100, 6000, 6000 and 5800 g polyketone/(g Pd h), respectively under conditions of 6.5 MPa and 110 degreesC. The effects of MA(n) component were also investigated; the results showed that suitable MA(n) was probably a metal compound of weakly coordinating anions; Mn+ probably coordinated with Cl- more strongly than palladium(II); the stability of chelate complex (c) of Mn+ with DPPP was probably between mono-chelate ring and bis-chelate ring complexes of palladium(II) with DPPP. This new highly efficient catalyst system had the advantages of having high stability and low cost.