An experimental analysis of HCCI by port injection of DME was carried out on an engine retrofitted from a diesel engine. The in-cylinder pressure with and without knock were processed by discrete wavelet transform. Wavelet energy of different levels were calculated and analyzed to obtain DME knock characteristics. The analysis shows that the energy distributions with and without knock display different characteristic. Without knock, the energy value of the detail D3 at level 3 is the largest, and the value of the detail D1 at level 1 is the largest with knock. With the increase of engine speed, or adding LPG into DME injected in the port have little effects on energy distributions. The knock intensity, however, plays a key role in determining their distribution. The more intensified the knock is, the larger the energy values of D1 goes. The energy value of D1, D2 and D3 can be used to detect knock.