SiC with d0 ferromagnetism is thought to be one of the most important materials in the spintronics field, and it has received widespread attention. In this paper, Al: SiC magnetic powder is fabricated by high temperature calcination method with the protection of Ar gas. X-ray diffraction results show that the obtained powder is of 6H-SiC phase, and Al is proposed to enter into the 6H-SiC crystalline. Raman results show that Ar gas plays a crucial role in impeding the SiC from decomposing at high temperature. With the protection of Ar gas, it maintains round shape after calcination about 2200℃, no any other peakis detected in the Raman spectrum. Without the protection of Ar gas, SiC particle would decompose into graphite, and the instinct peak of graphite is detected in the Raman spectrum. Energy dispersive spectrometer results show that there is 0.96 at% Al in the powder. The obtained powder shows magnificent magnetic hysteresis loop and large coercive force. Its saturation magnetic moment reaches 0.07 emu/g after calcination at 1800℃. Its coercive force reaches a maximum after calcination at 2000℃, while the saturation magnetic moment is 0.012 emu/g. With the rise of calcination temperature, the magnetism of the powder changes from diamagnetism to ferromagnetism. But when the calcination temperature rises to 2200℃ or more, it would change back to diamagnetism. The phenomenon of ferromagnetism disappearing is similar to that in ZnO as reported. The total quantity of magnetic impurities(Fe, Co, Ni) is evaluated to be less than 5 ppm. Saturation magnetic moments arising from these impurities can be calculated to be less than 10-5 emu/g according to the reported results, which is impossible to affect the accuracy in the experiment. Thus it is proposed that the ferromagnetism originates from the doping of Al in SiC powder. To understand the origin of the observed magnetism, we carry out first principles calculations based on spin polarized density functional theory. All the calculations are performed by using the generalized gradient approximation in the form of the Perdew-Burke-Ernzerhof function, which is implemented in the Viemma ab initio simulation package. A supercell consisting of 331 unit cells of 6H-SiC containing one AlSi-VSi, corresponding to a defect concentration of 0.93 at%, is built for calculations. The origin of its ferromagnetism is studied, and its spin situation in the space is mapped. The results show that the combination of Al and vacancy leads to a local magnetic moment of 1.0 B, and magnetic coupling is steady in the c axis direction. It is found that the p electron of carbon is the origin of the net spin.