Four　prism　concrete　specimens　with　same　height,　same　width,　but　different　lengths　is　simulated　in　using　ABAQUS　FEM　analysis　software.　The　simulation　work　is　conducted　under　monotonic　axial　compression　loading,　and　its　purpose　is　to　research　the　size　effect　of　prism　concrete　specimens,　primarily　to　study　the　effect　of　specimen　length　on　the　axial　compressive　strength.　The　stress-strain　relation　of　the　specimen　is　analyzed.　The　results　indicate　that,　for　different　lengths　of　specimens,　their　axial　compressive　strengths　are　quite　different　and　the　size　effect　on　them　is　much　obvious.　The　compressive　strength　improves　gradually　with　the　increasing　of　length,　and　size　effect　on　transverse　and　vertical　constraint　stress　of　core　concrete　area　is　much　distinct　as　well.　Meanwhile,　it　can　be　obtained　that　the　standard　design　method　of　the　structures　and　components　is　conservative　and　of　adequate　security.