By　analyzing　some　misunderstandings　about　the　principle　of　effective　stress　for　saturated　soil,　the　physical　meaning　of　effective　stress　of　soil　is　given　interaction　between　soil　based　on　the　particle　and　water.　The　effective　stress　of　soil　is　the　effective　load　transferred　between　the　soil　particle　skeletons　in　a　unit　section　and　equal　to　the　total　stress　minus　the　pore　water　pressure.　By　using　the　mechanics　balance　equation　of　soil,　on　the　one　hand,　the　Terzaghi　principle　of　effective　stress　for　saturated　soil　is　proved　and　its　sphere　of　application　is　analyzed,　on　the　other　hand,　the　Bishop　principle　of　effective　stress　for　unsaturated　soil　is　proved　and　the　physical　meaning　of　parameter　χ　is　explained.　By　combining　the　two　principles　in　the　same　mechanics　frame,　it　is　illustrated　that　the　Terzaghi　principle　of　effective　stress　for　saturated　soil　is　one　special　case　of　Bishop　principle　of　effective　stress　when　the　saturation　level　χ=1　in　theory,　and　the　sphere　of　application　of　the　Bishop　principle　of　effective　stress　for　unsaturated　soil　is　also　analyzed.　The　researches　show　that　by　analyzing　the　principle　of　effective　stress　from　two　aspects　of　loading　and　soil　properties,　the　physical　meaning　of　effective　stress　is　explained　clearly,　and　the　mechanism　of　the　influence　of　pore　water　pressure　mechanics　characters　of　soil　is　also　proposed.