Cracking monitoring provides an important way to evaluate structural safety. Cracking monitoring method based on distributed optical fiber sensor can effectively avoid undetected phenomenon existed in point-wise test method, and it can run automatically. A theoretical model of cracking monitoring based on obliquely intersected fibers and Brillouin Optical Time Domain Analysis (BOTDA) is introduced, and the numerical equation among fiber strain, crack width and crack angle is established by the calibration experiment. The experiments are also carried out to study the performance of different kinds of cracking sensors with different gauge length and pretension. The quantitative relationship between optical fiber measured strain and crack (including width and angle) is established for the 300 millimeter crack sensor. The cracking simulation test proved that this new method can effectively record the width and the angle of cracks during the whole cracking process.