Light-stimulated　optoelectronic　synaptic　devices　are　fundamental　compositions　of　the　neuromorphic　vision　system.　However,　there　are　still　huge　challenges　to　achieving　both　bidirectional　synaptic　behaviors　under　light　stimuli　and　high　performance.　Herein,　a　bilayer　2D　molecular　crystal　(2DMC)　p-n　heterojunction　is　developed　to　achieve　high-performance　bidirectional　synaptic　behaviors.　The　2DMC　heterojunction-based　field　effect　transistor　(FET)　devices　exhibit　typical　ambipolar　properties　and　remarkable　responsivity　(R)　of　3.58x10(4)　A　W-1　under　weak　light　as　low　as　0.008　mW　cm(-2).　Excitatory　and　inhibitory　synaptic　behaviors　are　successfully　realized　by　the　same　light　stimuli　under　different　gate　voltages.　Moreover,　a　superior　contrast　ratio　(CR)　of　1.53x10(3)　is　demonstrated　by　the　ultrathin　and　high-quality　2DMC　heterojunction,　which　transcends　previous　optoelectronic　synapses　and　enables　application　for　the　motion　detection　of　the　pendulum.　Furthermore,　a　motion　detection　network　based　on　the　device　is　developed　to　detect　and　recognize　classic　motion　vehicles　in　road　traffic　with　an　accuracy　exceeding　90%.　This　work　provides　an　effective　strategy　for　developing　high-contrast　bidirectional　optoelectronic　synapses　and　shows　great　potential　in　the　intelligent　bionic　device　and　future　artificial　vision.