Hello ngochien. Thank you for your comment on our Roger little air purifier. Unfortunately, it is not possible to fit a textile pre-filter to the Roger little of the previous generation. The air purifiers of the current generation are automatically equipped with a textile pre-filter. If you have any further questions, please feel free to contact us directly by email at service@stadlerform.com or by phone on 041 720 48 48. Best regards, your Stadler Form Team