The invention discloses a photovoltaic string welder and a welding method of battery strings. The photovoltaic string welder comprises a box body and a conveying belt penetrating the interior of the box body and used for conveying battery pieces; interconnector welding areas corresponding to the positions of the battery pieces, and bus-bar welding areas corresponding to the positions of interconnectors extending out of the battery pieces are included in the box body; first infrared lamp pipes used for welding the interconnectors to the battery pieces are arranged in the interconnector welding areas; and second infrared lamp pipes used for welding bus-bars to the interconnectors are arranged in the bus-bar welding areas. According to the photovoltaic string welder and the welding method of the battery strings, the interconnectors and the bus-bars at the two ends of the battery strings can be synchronously welded, welding spot connection of the interconnectors is not needed, the manual welding spot number is greatly reduced, the manual operation workload is relieved, and the welding efficiency is greatly improved.