For　the　analysis　of　how　much　information　can　be　hidden　in　digital　image,　a　content　adaptive　watermarking　algorithm　is　presented　and　watermarking　capacity　is　discussed　in　this　scenario.　The　relation　between　watermarking　capacity　and　detection　bit　error　rate　is　also　derived.　Research　result　shows　that　watermark　detection　error　is　mainly　influenced　by　the　watermark　mean　energy　and　watermarking　capacity.　The　detection　error　rises　with　the　increase　of　watermarking　capacity.