A simple analytical optical power transfer function is derived in frequency domain for intensity modulation systems with fiber loss, chromatic dispersion and self-phase modulation. Expression to calculate the distortion of arbitrary optical power waveform is obtained with distinct physical meaning of each term. The results show that the induced power variation is proportional to the square of modulation frequency (SOMF) at low frequency region, while proportional to the sine of SOMF at high frequency region. The analytical result agrees well with numerical simulation under weak modulation and can correctly predict the time-domain waveform under strong modulation such as non-return-to-zero in intensity modulation/direct detection systems.