How to encode/decode PDUs of protocols is always a time-wasted complicated work in the phase of developing the systems for the telecommunication or internet networks. As ASN.1's well known efficient way to describe the complicated protocols, many protocols PDUs of the presentation layer or the application layer (e.g TCAP, INAP) in the telecommunication network and internet have been represented in ASN.1. One of the key protocols of SS7 network in the legacy telecommunication network is the ISDN user part (short named as ISUP). Its development and extension have an important influence on the related voice telecommunications. However, no ASN.1 presentation file of the ISUP PDUs appears in the ITU-T specifications. One important reason is the special structure of ISUP PDUs against ASN.1 BER which forms the base for the ASN.1 presentation of any protocol PDUs like those PDUs of the protocols (e.g TCAP, INAP, MAP, SNMP). Because of the wide usage of the ASN.1 and the importance of the ISUP, it makes sense to parse the ISUP PDUs with the ASN.1. What's more, there are many open free tools available for parsing to compile ASN.1 into other languages easily, which can ease developing ISUP PDUs parsing tools. In this paper, we present a new effective approach to encode/decode it in the same way as the other protocol PDUs presented in ASN.1. By this approach, we have developed a library available under Linux for parsing ISUP PDUs via ASN.1.