JPEG2000 is the newest image coding standard which provides flexible scalability. Designing an encryption system for scalable JPEG200 codestream is becoming a new challenge. Recently, Wu et al.[13] proposed a flexible access control scheme for JPEG2000 codestream. However, it needs key regeneration whenever users change the preferred progression order. In this paper, we propose an improved flexible access control scheme for JPEG2000 codestreams based on Multi-Dimensional Hash Chains (MDHC), in which the keys need not to be regenerated whenever different progression order of packets is required. In particular, we employ multiple hash chains to generate a series of keys to encrypt each packet in the codestream. Furthermore, users can access the encrypted codestream according to any authorized combinations of resolution, quality layer and precinct with as few as possible keys. Also, our proposed scheme is secure against collusion attack as well as unauthorized access.