Based on the ray tracing method，the implicit expression of the output spectrum of the extremely short external cavity semiconductor laser (ESECSL) has been deduced for the first time. As a result，the output spectrum and P-I characteristic of ESECSL have been investigated. The results show that: when the length of external cavity changes by an amount of the order of wavelength，the P-I characteristic of ESECSL will change obviously. The lasing wavelength of ESECSL will hop periodically in the range of 10nm with the variation of the length of external cavity. Especially，the hopping range of the lasing wavelength will reach the maximum for the external cavity length varying within the range of 40—70μm. The simulations are well in accordance with the experimental reports.