A heat treatment process for crystalline silicon solar cell sheets, comprising: a) sifting out, from crystalline silicon solar cell sheets on which printing and sintering are performed, cell sheets with conversion efficiencies lower than 18 percent and filling factors higher than or equal to 70 percent (S21, S307); b) performing low temperature annealing on the sifted cell sheets (S22, S308) to improve the conversion efficiencies of the sifted cell sheets, wherein a temperature for the low temperature annealing is lower than that of the sintering; characterised by comprising: c) performing a sorting test on the cell sheets on which the low temperature annealing is performed (S23, S309), to sift out cell sheets for which the filling factors are decreased; d) re-sintering the cell sheets sifted out in step c) (S24, S310) to increase the filling factors of the cell sheets, wherein a temperature for the re-sintering is same as that of the sintering; and e) performing a sorting test on the re-sintered cell sheets (S25, S311) to sift out the cell sheets with the conversion efficiencies lower than 18 percent and returning to step b), until most or all of the sifted cell sheets have the conversion efficiencies higher than 18 percent and have the filling factors higher than or equal to 70 percent. a) sifting out, from crystalline silicon solar cell sheets on which printing and sintering are performed, cell sheets with conversion efficiencies lower than 18 percent and filling factors higher than or equal to 70 percent (S21, S307); b) performing low temperature annealing on the sifted cell sheets (S22, S308) to improve the conversion efficiencies of the sifted cell sheets, wherein a temperature for the low temperature annealing is lower than that of the sintering; characterised by comprising: c) performing a sorting test on the cell sheets on which the low temperature annealing is performed (S23, S309), to sift out cell sheets for which the filling factors are decreased; d) re-sintering the cell sheets sifted out in step c) (S24, S310) to increase the filling factors of the cell sheets, wherein a temperature for the re-sintering is same as that of the sintering; and e) performing a sorting test on the re-sintered cell sheets (S25, S311) to sift out the cell sheets with the conversion efficiencies lower than 18 percent and returning to step b), until most or all of the sifted cell sheets have the conversion efficiencies higher than 18 percent and have the filling factors higher than or equal to 70 percent. c) performing a sorting test on the cell sheets on which the low temperature annealing is performed (S23, S309), to sift out cell sheets for which the filling factors are decreased; d) re-sintering the cell sheets sifted out in step c) (S24, S310) to increase the filling factors of the cell sheets, wherein a temperature for the re-sintering is same as that of the sintering; and e) performing a sorting test on the re-sintered cell sheets (S25, S311) to sift out the cell sheets with the conversion efficiencies lower than 18 percent and returning to step b), until most or all of the sifted cell sheets have the conversion efficiencies higher than 18 percent and have the filling factors higher than or equal to 70 percent.