New methods for the determination of magnesium in fertilizer by microwave digestion using inductively coupled plasma optical emission spectrometry(ICP-OES method) and atomic absorption spectrometry(AAS method) were established. The results of the new methods and the method in the national standard GB/T 19203—2003 were studied by t-test. Two compound fertilizer standard materials and 15 fertilizer samples from different regions，different magnesium sources and different magnesium contents were determined. The t-test results showed that there was no significant difference in the extraction results between the microwave digestion method and the nitric acid-perchloric acid electric heating plate digestion method in GB/T 19203—2003；there was no significant difference in the measurement results among the ICP-OES method，AAS method and the volumetric method in GB/T 19203—2003. The recovery rate and precision of ICP-OES method and AAS method were superior to those of the volumetric method. For microwave digestion-ICP-OES method，the standard material recovery rates were 110% and 108%，respectively，and the relative standard deviation of 6 parallel determinations was 0.80%. For microwave digestion-AAS method，the standard material recovery rates were 109% and 108%，respectively，and the relative standard deviation of 6 parallel determinations was 1.29%. Compared with AAS method，the ICP-OES method had better precision and standard curve determination coefficient，wider linear range of the standard curve，and was more convenient and environmentally friendly in the experimental process，which made it more suitable for fertilizer samples.