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Slow extraction from synchrotrons is widely used for delivering hadron beams to many physical experiments as well as hadron cancer therapy based on charged particle deposition on tumors. The extracted beam is also referred to as spill. The temporal uniformity of the slow extracted beam, also known as spill uniformity, is an important indicator of spill quality. A uniform beam intensity throughout the beam spill time is usually required in these physical experiments and hadron cancer therapy. In order to meet the requirements of these applications, the spill uniformity from the accelerator must be improved. In practice, the spill uniformity is degraded due to the influence of various factors, the intensity of the extracted beam fluctuates, forming spill ripple. How to suppress the spill ripple and improve spill uniformity has always been an important research direction in the field of slow extraction from synchrotrons. In order to optimize the spill uniformity, it is first necessary to determine the source of the nonuniformity of the spill, that is, the spill ripple. The spill spectrum plays a key role in analyzing the spill ripple, the conventional spill spectrum analysis method judges the main source of spill ripple through the spectrum amplitude, and the conventional method cannot correctly reflect the influence of the spill ripple with a frequency bandwidth. In this paper, a method for quantitative analysis of the spill spectrum was proposed. According to this method, spill ripples caused by different reasons can be classified, and the contribution of different types of spill ripples to nonuniformity can be quantitatively calculated. The quantitative comparison of the impact of different types of ripples can be realized. This method was applied in the optimization of the spill uniformity at Xi’an 200 MeV proton application facility synchrotron. After analysis, it is found that the main spill ripple of the XiPAF synchrotron came from the magnet power supply ripple and low-frequency ripple. In the experiment, the influences of magnet power supply ripple and low-frequency ripple are suppressed respectively. The influence of the magnet power supply ripple is suppressed by optimizing the spectrum of the RF-KO signal to reduce the particle density near the separatrix. And the low-frequency ripple is suppressed by the feedback control of the amplitude of the RF-KO. After optimization, the spill uniformity of the XiPAF synchrotron is significantly improved, and the standard deviation of the extracted beam intensity divided by the average could reach about 10% at a sample rate of 1.3 kHz.