For concentration III-V tandem solar cells, very great tunneling peak current would be essential. Basing on Esaki diode tunneling theory，this paper simulated the impact of impurity density upon the peak current in a homo-junction GaAs/GaAs tunneling junction. And the influence of band-gap graded on tunneling peak current of solar cell was further taken into consideration, which would play a practical role in designing tunneling current configure ration of multi-junction solar cell. The results shows that high impurity density and band-gap graded have a great effect on the tunneling peak current. Typically, with doping concentration 5×10¹⁹cm⁻³ and the influence of the band-gap graded, 1903.5A/cm² of the simulated GaAs/GaAs tunnel junction peak current has been obtained. Correspondingly, a practical value 139.2A/cm² of the tunneling current was developed, which could satisfy the conditions of work under the 1000 suns.