On the basis of SPAD model for nutrition diagnosis of nitrogen fertilization, we determined the optimal nitrogen rate at the different growth stages of cotton with drip irrigation under mulch. Combining with relevant research at a single irrigation time and acreage of round irrigated area, we definited the fertilizer time of different specifications fertilizer disc. The capacity of the device with combined application of nitrogen fertilizer was determined by comparing with distribution and application of combined fertilizer at unit time. The results showed that the rate of fertilization per unit time for the device was related to water yield from any single well and nitrogen fertilizer in the whole cotton growth period, and the device were unable to meet the needs of topdressing when it used fertilizer disc of sizes with 100g, 200g and 300g. Furthermore, fertilizer disc of sizes with 400g and 500g could only meet topdressing nitrogen level of cotton under condition of the specific water yield from a single well. And according to our research described above, we explored the approach to optimize and improve device.