Objective  To discuss the critical issues of the dynamics of UV-light-photoinitiated cross-linking in corneal collagen （CXL） and to confirm the dynamics of riboflavin （vitamin-B2） absorption under UV light. Methods  Coupled dynamic equations are numerically solved and analytic formulas are derived for three critical parameters： the safe depth （z*）， the safe dose （E*） and the cross-linking time （t*）. Time-dependent absorption of UV light due to the depletion of the initiator is measured and shown by a dynamic spectrum of riboflavin. The critical issues of CXL are explored by seven parameters： the extinction coefficient， concentration， the penetration depth of the riboflavin， the UV light intensity and dose， irradiation duration， and corneal thickness. Results The safe dose （E*） has a wide range from 2.3 to 8.2 （J/cm2） for riboflavin concentrations of 0.1% to 0.2% and penetration depths of 0.02 to 0.04 cm. It is shown by mathematical modeling that a higher light intensity and extinction coefficient lead to shorter t* for a given cross-linking depth， while t* increases with corneal thickness （z*）. The safety depth decreases as a function of the extinction coefficient and initiator concentration. Conclusion  A new cross-linking protocol is suggested based on new findings， which include the safe depth （z*）， the safe dose （E*）， the cross-linking time （T*）， and the safe riboflavin concentration.