Abstract: Karst problem is a big puzzle to tunnel engineering professional for a long time, especially for karst geology forecasting in constructing tunnel, it is worth to research the accuracy and reliability of forecasting. The research considering-line is put out as this, the following aspects associate with each other, that is, macrograph with microscopic view, numerical modeling with systematic analysis, and qualitatively analysis with quantitatively analysis, thus the application research of forecasting method is mainly developed by using fractal theory. On the basis of fractal theory, according to the research production of fractal characteristic on microscopic fracture and pore, the basic conception, karst unit, is put forward, The fractal growing law of karst unit is described, macro-and micro-karst types are researched in fractal characteristics. According to the the calculated Comprehensive Index of Karst Fractal Assessment, the karst development degree is divided as five steps. Tongzilin tunnel is as an example of macro-and micro-karst types outside or underground the tunnel, the fractal index are calculated and the karst development degree is evaluated, all these have built the theory foundation on following karst geology fractal prediction before the working face of tunnel.