A multi-symbol detection (MSD) method based on the approximation of ML decoding for detecting the signal modulated by PPM in unknown UWB channels is proposed. The explicit expression of the achievable rates of PPM with MSD is derived, from which we study the relationship between the achievable rates, SNR and channel bandwidth. The numerical results show that multi-symbol detection significantly improves the achievable rates over conventional non-coherent detection. When channel bandwidth tends to infinity, as long as symbol number N keeps up with the resolvable multipath number L, the achievable rates will tend to AWGN capacity.