The effect of cooling field HFC on the exchange bias field HEB in a spontaneous lamellar ferromagnetic/antiferromagnetic phase separated Y0.2Ca0.8MnO3 has been studied. It is found that with increasing HFC from 1to6T the value of HEB decreases by 37% at 2K and is inversely proportional to the ferromagnetic layer thickness tFM. This suggests that the tuning of HEB by HFC in Y0.2Ca0.8MnO3 arises from the variation of tFM with HFC. This phenomenon is essentially different from other types of magnetic tunings.