In order to achieve high and stable production and improve oil recovery，waterflooding plan is used in the most sandstone oilfields of China. However，most oilfields have entered the production decline stage with water breakthrough and water cut increasing. In order to slow down the decline and improve the development status and the oil recovery，well pattern infilling and interlayer adjustment were carried out. It is well known that the recoverable reserves and remaining recoverable reserves of oilfields are the basis of development programs and adjustment programs. At present，the recoverable reserves and the remaining recoverable reserves of Chinese oilfields are evaluated by international assessment company. The method used is the classical production decline method proposed by Arps in 1945，and mainly based on the exponential decreasing method. Based on the generalized Arps decline form，a method was proposed to evaluate the recoverable reserves， remaining recoverable reserves，reserves production ratio（RPR）and decline rate before and after the infilling adjustment of the oilfield. The practice application indicates that the technical recoverable reserves and economical recoverable reserves have been increased by 409.1×104 and 412.8×104 t respectively after infilling adjustment in Maling Oilfield.