Photosynthetic indexes and micrometeorologic conditions of four weed species Echinochloa colonum(L.)Link,Chloris virgata Swartz,Setaria glauca(L.)Beauv.,Digitaria sanguinalis(Linn) Scop.were detected during their fructification period.The differences of photic utilization and water utilization were concluded respectively.The daily change of net photosynthetic rate(Pn) of four weed species followed double apex curve.The first apex all appeared at 10:00,and the second one emerged at 15:00(except) Echinochloa colonum whose apex all appeared at 13:00.The Pn of Echinochloa colonum whose apex value is the highest had the weakest influence of micrometeorologic conditions.The highest Pn of Echinochloa colonum exceeded Chloris virgata,Digitaria sanguinalis,Setaria glauca 16.6%,41.3%,53.4% respectively during morning.However Chloris virgata whose Pn was the highest in the afternoon was extremely influenced by temperature and light.The highest Pn of Chloris virgata exceeded Digitaria sanguinalis,Setaria glauca,Echinochloa colonum,31.8%,9.8%,31.2% respectively during afternoon.Echinochloa colonum had the highest water utilization efficiency.