Tube voltage volues used to be determined by doing test in X-ray flaw detection of titanium steel welds,which was considered low efficiency and costly. The author has fornd out the relationship of tube voltage value with thickness of samples and transillumination distances under different modes and conditions, and established an empiric formula for calculating tube voltages.The conparasion analysis of calcuated values and experimental values shows that the empiric formula is simple and reliable. The tube voltage calculation equation and related datum,which are suitable for several commonly used X- ray flaw detectors and photographic films at home,are persented in this paper.