To better understand the mechanism of weed occurrence and suppression effect of crops on weeds, we conducted a 4\|year field experiment in 4 crops under different tillage and fertilization regimes in Ansai, Shaanxi, China. The species composition, seed density, diversity and the similarity were measured in soil seed bank of soybean (Glycine max), corn (Zea mays), Red Bean (Semen Phaseoli ) and potato (Solanum tuberosum). The tillage and fertilization treatments in each crop included till\|chemical fertilizer (CF), till\|organic manure (CM), till no fertilizer (CN), no\|till chemical fertilizer (NF), no\|till\|organic manure (NM), and no\|till\|no fertilizer (NN). The results showed that: (1) There were 12 weed species in 1965 seedlings from 24 soil samples in 4 crops. These species belonged to 7 families and 12 genuses and 94% of them were annual weeds. Bangtou grass (fugax nees ex steud), amaranth (Acalypha australis), large crabgrass (Digitaria sanguinalis） and Poa（Poa sphondylodes）were the dominant species and accounted for 87%; (2) In 0-20cm soil of different treatments, weed density ranged from (282.9 ± 63) to (7482.5 ±1078.3) seeds·m-2. There were significant differences (P<0.01) in weed density among crops, tillage systems, and fertilization regimes. The ranking of weed seed density was Red Bean plot> potato plot > soybean plot > corn plot among 4 crops, tillage plot > no\|tillage plot between 2 tillage treatments, and organic fertilizer plot > Fertilizer plot > no fertilizer plot among fertilization treatments; (3) Soil seed bank had the highest weed density in NM Red Bean and the lowest density in NN corn. The rich index was the highest (2.30) in CF soybean and the lowest (0.29) in NN Red Bean. The diversity index was the highest in NN soybean (5.56) and the lowest in CF Red bean (045). The ecological advantage of NF corn was the highest (1.35) and that of CF soybean was the lowest (0.17). Among the fertilization treatments, the similar coefficient of soil seed bank ranged from 0 to 0.63 in tilled system and from 0.67 to 0.92 in no\|till system. As a result, corn and soybean were optimal crops to inhibit weeds in the dry land farming region of the Loess Plateau, and the best treatments were NM corn, NF soybeans and NM soybean. The ecological benefits of these crops include inhibiting the spread of weeds competitive to crop, maintaining high diversity and equilibrium of weeds, decreasing weed density in soil seed bank and optimizing soil environment.