Research on microstructure and behavior of natural biomaterials contributes to understanding the biomimetic design. Yakbone bioceramic’s cross section with innumerable voids was observed via SEM approach, then a digital image processing technique was adopted to analyze the images. It is supposed that the pores can be classified into n series by different size level. A non-linear function between the pore size and series number got fitted which indicates a regression trend to zero as n rising to a large number. A non-linear convergent analysis of the function results in the porosity of porous microstructure of yakbone bioceramic. Furthermore, a plane helices characterization model was assumed to descript the multilevel voids distribution, then a series of helices parameters were designed for bionic yakbone porous microstructure with porosity tolerance of 2.5%. This parameter description of yakbone, a porous bioceramic with high strength and light weight, contributes to further research on the relationships between mechanical property and microstructure.