Is the horoscope of Jiwei year, Dingmao month, Guiyou day and Guihai hour pure yin? How to solve it?

Yes.

In ancient China, 10 heavenly stems and 12 earthly branches were used to keep time. The 10 heavenly stems are A, B, B, D, Wu, Ji, Geng, Xin, Ren and Gui. The 12 earthly branches are: Zi, Chou, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu and Hai. It also stipulates that Jia, Bing, Wu, Geng, and Ren are the yang heavenly stems; Zi, Yin, Chen, Wu, Shen, and Xu are the yin earthly branches. So the horoscopes you mentioned are pure yin horoscopes.