Developing overall strength in your body, in particular the deep stability muscles of your  hips, pelvis and spine which forms your core will increase your back strength.  These stability muscles are engaged in all Pilates exercises. This helps to restore back strength, and maintain back strength. In addition Pilates helps develop spinal mobility and flexibility.
If you do have concern with your back, it is advisable to check with your GP to ensure Pilates is right for you at this time.