The　unsaturated　loess　exists　widely　in　Northwest　China.　It　has　become　an　attention　question　because　it　has　specialty　engineering　property　such　as　the　collapsibility　and　low　bearing　capacity　and　come　into　being　strictness　harm　to　engineering.　The　process　of　form　and　development,　microstructure　property,　mineral　and　chemistry　constitution　of　loess　are　analyzed　in　detail.　At　the　same　time,　the　relations　of　the　collapsibility　with　inundation　time,　water　content,　saturation　and　pressure　are　obtained.　It　affords　the　foundation　of　engineering　design　and　construction.