In-situ　particle-reinforced　composite　solder　was　considered　as　an　effective　method　to　enhance　the　properties　of　lead-free　solders.　In-situ　Cu6Sn5　particles　were　added　into　the　eutectic　Sn-3.5Ag　solder　to　form　in-situ　metal　particles　enhanced　SnAg-based　composite　solder.　The　microstructure　and　mechanical　properties　of　the　composite　solder　joint　during　reflow　and　isothermal　aging　were　investigated,　and　the　fracture　mode　of　composite　solder　joint　and　the　role　of　in-situ　Cu6Sn5　particles　were　analyzed.　Experimental　results　show　that　the　microstructure　and　dimensions　of　in-situ　Cu6Sn5　particles　in　composite　solder　matrix　have　changed　during　reflow　and　isothermal　aging.　So　the　shear　strengths　of　composite　solder　joints　have　influenced　by　the　sizes　of　in-situ　Cu6Sn5　particles.　Besides,　the　deformation　of　composite　solder　joint　is　dominated　by　shear　bonding　mode,　and　the　in-situ　Cu6Sn5　reinforced　particles　can　retard　the　fracture　path　of　shear　bonds.