Lateral　current　spreading　is　experimentally　and　theoretically　investigated　in　stripe　laser　diodes.　The　influence　of　current　spreading　length　to　threshold　current　density　is　quantitatively　analyzed.　The　factors　of　current　spreading　length　that　residual　thickness,　resistivity　of　upper　cladding　layer,　injected　current　density　are　numerically　calculated.　It　is　found　that　effective　current　rate　declines　with　the　injected　current　increasing　and　resistivity　affects　the　current　spreading　length　dramatically.　A　critical　injected　current　density　exists　below　which　the　slope　efficiency　increases　with　the　injected　current　density　increasing.　Above　this　critical　value,　the　slope　efficiency　declines　with　the　injected　current　density　increasing.　This　critical　value　helps　to　find　the　best　work　current,　which　can　extend　the　lasers＇　life.