The horizontal air convection and complex ground conditions lead to the uncertainty increasment of the calculation of atmospheric thermal radiation. The measurement of infrared radiation characteristics during the initial part of target launching is helpful for target recognition. Therefore, it is very important to study the atmospheric transmittance and thermal radiation in the horizontal direction to improve the accuracy of the infrared radiation characteristics measurement of the target. Based on the problems mentioned above, the "horizontal temperature profile" model of the same layer atmosphere is established referring to the vertical profile theory firstly. Then the formula of calculating the atmospheric transmittance and thermal radiation in the horizontal direction is deduced, and the experiment verification is carried out. Finally, the results of MODTRAN software calculation and experiment measurement of the atmospheric thermal radiation are compared. The results show that the accuracy of traditional software is 8.65% and the accuracy of the proposed calculation method is 4.91%. The experiment proves that the calculation method is correct in the horizontal direction. The validity of the calculation method is demonstrated by formula derivation and experimental verification. A new idea is provided for the calculation of the atmospheric transmittance and thermal radiation in the horizontal direction.