A fission neutron detector system with frontward-extended lead slot collimator has been developed and tested. By designing the slot collimator and developing large area PIN detectors as well as large area fissionable target with thin Be backing, this detector system can reach high neutron sensitivities up to 10-16C·cm2, which is about 4-5 orders of magnitude higher than that of existing fission detectors，and high n/γ　discrimination ratio up to 30.