The experiment was conducted at Yucheng comprehensive experimental station to study dynamic variety of soil organic matter and available nutrient content in winter wheat and summer maize a year under the conditions of straw mulching and irrigation in 2002-2003, the results showed it was important for straw mulching to increase organic matter in winter wheat and summer maize a year planting system. Under the conditions of without straw mulching, irrigation could decrease organic matter content, but under the conditions of straw mulching, the effects were unobviously. In growth stages of winter wheat, irrigation could decreased alkali-hydrolyzable. N content, and straw mulching could decrease the content greatly, the reason for this was eluviation. Available P was increased greatly under the conditions of straw mulching, and irrigation could increase the content greatly. Available K was increased greatly under the straw mulching conditions. In growth stage of summer maize, the treatments which irrigated in winter wheat growth stages could increase Available K content greatly.