Spin-squeezed state is a many-body entangled state of great interest for precision measurements. Although the absolute sensitivity at the standard quantum limit is better for a larger atom number, the greater dominance of classical noises over atom projection noise makes it harder to achieve spin squeezing. Here, we show both theoretically and experimentally that adiabatic pulse control of the pump field in state preparation is indispensable to sufficient noise suppression, which is the prerequisite for spin squeezing. This technique is generally applicable to spin-squeezing experiments involving a large ensemble and is thus of significance for quantum metrology applications.