Mainlobe of regular beamforming changes with frequency and spatial direction. In order to achieve constant mainlobe in both frequency and steering domain, a wideband Frequency-Steering Invariant Beamforming (FSIB) algorithm based on constrained optimization is proposed. Firstly steering invariant constraints on reference frequency are realized, and then frequency invariant constraints on different mainlobe directions are realized. Finally constraints are transformed to standard Second-Order Cone Programming (SOCP) forms and solved with SeDuMi. A wideband DOA estimation method using FSIB is also proposed. Simulations show the proposed FSIB algorithm can realize constant mainlobe on different frequencies and directions. Compared with method using Frequency Invariant beamforming (FIB), DOA estimation using FSIB has better performance in resolution and accuracy.