A theoretical model is presented for the prediction of sound radiated from an unbaffled long enclosure with ground effect. This geometrical arrangement forms an idealized representation of traffic facilities such as tunnels and railway stations where sound propagates along the enclosures and radiates to the outside through the openings at both ends. The sound fields inside and outside the enclosures should be accurately predicted and thoroughly analyzed so as to determine an appropriate noise control strategy. In this paper, the Fourier transform technique and the mode matching method are firstly applied to transform the intractable boundary value problem into a scalar modified Wiener-Hopf equation. The solution of which contains infinitely many unknowns satisfying infinite linear algebraic equations susceptible to numerical treatment. Good agreement is found between the solutions obtained by the finite element method (FEM) and the proposed technique in a wide frequency range. Then, the far-field directivity patterns of the outside acoustic fields are emphatically illustrated and the formation mechanisms of lobes, zeros and the after radiation are discussed in details. In the end, the advantages and the application prospects of the proposed method are summarized.