If your form is correct and you are still having issues with low back pain on a deadlift, then there might be a mobility issue. What could happen? Your hamstrings might be too tight and that is pulling your spine into flexion so you are unable to keep the “back flat”.
There are two exercises that will help place your hips in a good position.
Banded Hip Stretch – with the band anchored, place the band at the hip crease. Leg that is banded is slightly forward. This is forcing your femur to go back and allow a stretch in the hamstrings. Lean body forward. Slowly bend and straighten the knee not allowing the knee to dive in. You will feel a stretch from your calves all the way to your hamstrings. Hold 2 minutes each side.
Banded Hip Hinge – Step forward in the same banded position. Practice the deadlift motion. Hip hinge, flat back and squeeze glutes at the top. Don’t bend at the low back. Complete 10-15 reps each side.