This paper presents the principle of a new composite (C2H4)n-PIN sandwiched fusion neutron detector, which uses 2mm (C2H4)n as sensitivity enhancement medium, and add circuit to merge the output signals. The main merits of the detector are: (1) greatly improved neutron sensitivity and better statistics; (2) improved n/γ discrimination; (3) implementing the signal summation without obviously affecting the time response. Characteristics of the detector, including the sensitivity to 14MeV neutron and 1.25MeV γ ray, n/γ discrimination, time response, and statistics are studied theoretically and experimentally.